I think problem is in this line
t.HCCDescription+ isnull(pp.HCC_Description,'')
for example if second filed is null then you are trying to concatenate the first field with null object, so that it's throwing that error.
I request you to please try below query
select distinct t.HICN,t.PatientName,t.PracticeName,ISNULL(Convert(varchar(100), t.HCCDescription)+ Convert(varchar(100),pp.HCC_Description)) as HCCDescription
from temptbl_CCMData t left outer join VwLatestPatientDetails vw on vw.hicnumber=t.hicn
left outer join icdhcc pp on pp.HCC=t.HCCDescription WHERE VW.taxid=593516436