While using a subquery inside the inner query, variable declared in outer query becomes inaccessible to inner queries inside it.how to resolve this error?

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C#

LINQ

My project has subqueries used inside nested Query. While using a subquery inside the inner query, variable declared in outer query becomes inaccessible to inner queries inside it.
Error found while debugging nested queries:
1. Unknown column 'Join3.appcategoryid' in 'field list'
2. Unknown column 'Extent1.taxonomyid' in 'where clause'

public static dynamic GetTaxonomies(int businessUnitId, int memberId, string businessUnitType)
       {
           using (var db = new ProtocolManagementDatabaseEntities())
           {
               var taxonomies = (from taxonomy in db.taxonomies
                                 join businessUnitTaxonomy in db.businessunittaxonomies 
                                 on taxonomy.taxonomyid equals 
                                  businessUnitTaxonomy.taxonomyid
                                 let TaxonomyID = taxonomy.taxonomyid
                                 where businessUnitTaxonomy.businessunitid == 
                                 businessUnitId
                                 select new
                                 {
                                     TaxonomyId = taxonomy.taxonomyid,
                                     TaxonomyName = taxonomy.taxonomyname,
                                     preferenceValue = db.userpreferences.Where(x => 
                                     x.entitycolumnvalue == TaxonomyID &&
                                     x.memberid == memberId && 
                                     x.entitytablename == "Taxonomy" && 
                                     x.sequencenumber == 5)
                                     .Select(x => x.isvisible).FirstOrDefault()

                                     AppCategory = (from appCategory in 
                                     db.appcategories
                                     join appCategoryTaxonomy in 
                                     db.appcategorytaxonomies on 
                                     appCategory.appcategoryid equals 
                                     appCategoryTaxonomy.appcategoryid
                                     let AppCategoryID = appCategory.appcategoryid
                                     where appCategory.parentappcategoryid == null &&
                                                   
                                 appCategory.appcategorylevel.Equals("AppCategoryName")
                                                    && 
                                 appCategoryTaxonomy.taxonomyid == TaxonomyID &&
                                 appCategoryTaxonomy.businessunitid == businessUnitId
                                 orderby appCategory.name ascending
                                 select new
                                   {
                                     AppCategoryId = appCategory.appcategoryid,
                                     AppCategoryName = appCategory.name,
                                     AppCategoryDeletionEnable = 
                                     db.appcategories.Where(x => 
                                     x.parentappcategoryid == 
                                      AppCategoryID).FirstOrDefault() != null ? true : 
                                     false
                                     SubCategory = from subCategory in db.appcategories
                                     let SubCategoryID = db.appcategories.Where(x => 
                                     x.appcategoryid == 
                                     subCategory.appcategoryid).Select(x => 
                                     x.appcategoryid).FirstOrDefault()
                                     where subCategory.parentappcategoryid == 
                                     AppCategoryID && 
                                     subCategory.businessunitid == businessUnitId
                                     orderby subCategory.name ascending
                                     select new
                                         {
                                           AppCategoryId = subCategory.appcategoryid,
                                           AppCategoryName = subCategory.name,
                                           Description = subCategory.description,
                                           IsGlobal = subCategory.isglobal,
                                           IsEnable = subCategory.isenable,
                                           ParentAppCategoryID = 
                                          appCategory.parentappcategoryid,
                                                                          
                                          SubCategoryDeleteEnable = 
                                          db.protocolappcategories.Where(x => 
                                          x.appcategoryid == SubCategoryID)
                                                                          
                                          .FirstOrDefault() != null ? true : false,

                                                                          
                                            AppCategoryLevel = 
                                            subCategory.appcategorylevel,
                                            IsExpand = false,
                                                                      }
                                                    }).ToList()
                                 }).ToList();
               return taxonomies;
           }
       }

What I have tried:

Error found while debugging nested queries:
1. Unknown column 'Join3.appcategoryid' in 'field list'
2. Unknown column 'Extent1.taxonomyid' in 'where clause'