Only Functions And Extended Stored Procedures Can Be Executed From Within A Function. Sp_executesql Is A Extended Stored Prod

i have created the folowing function but keep geting an error.

Only functions and extended stored procedures can be executed from within a function.

Why am i getting this error!

Create Function myDateAdd
(@buildd nvarchar(4), @avdate as nvarchar(25))
Returns nvarchar(25)
as
Begin
declare @ret nvarchar(25)
declare @sqlval as nvarchar(3000)

set @sqlval = 'select ''@ret'' = max(realday) from (
select top '+ @buildd +' realday from v_caltable where realday >= '''+ @avdate +''' and prod = 1 )a'

execute sp_executesql @sqlval
return @ret
end