Conversion failed when converting the varchar value to data type int. i cant resolve this error please suggest me a correct solution

ALTER proc  [dbo].[pGetCode] --'InvestmentGroup', 'Code', 'I',5 
(

@TableName  varchar(20),

@ColumnName varchar(20)='',
@key varchar(10),
@TotalDigits int 
)
as
begin
create table #temp (incode varchar(20));
declare @incode varchar(20);
declare @a varchar(10)=null;
declare @length int = 1;
declare @string varchar(10) ='';
declare @startposition varchar(20);
declare  @query varchar(100);

set @startposition=Len(@key)+1;


select @query='insert into #temp select   SUBSTRING( max('+@ColumnName+') ,'+@startposition+','+@TotalDigits+') as  int  from  '+ @TableName +'  where '+@ColumnName+' like '''+ @key +'0%'''
exec (@query)
select @incode = incode from #temp;
select @incode
drop table #temp;
end