SQL server + dynamic query + 'The data types nvarchar and bit are incompatible in the add operator.'

You're not converting [IsBirthdayVoucher] to VARCHAR, you're converting @var2 to VARCHAR.

Try:

SET @sql = ('Select ' + @var1 + ' + Convert(varchar, ' + @var2 + ') 
             From [VoucherType] 
             Where [DeletedBy] is Null AND [AutoID] = 1')