How to handle Box 24 of T3

This is a foreign business income that should be declared on line 135 which can only be filled through T2125. T2125 require few mandatory information in the identification section. If you are claiming foreign tax credits on T2209 for this income, then you do not qualify to file using Netfile. In that case, you can enter the income on T2125, print(ignore the error) the return and mail it.
If you  are not claiming the foreign tax credit, then remove the amount from T3, add it manually to T2125 and complete the form using the info described in the business guide .