Hello! 

Error 2123 arises only if you are filing your return more than three years after the end of the tax year.

To remove the EI overpayment amount on line 45000 of your return:

• Make note of the amount on line 31200 of your Schedule 1 (in the Documents section).
• Enter the amount from line 31200 in box 18 of your T4 slip (regardless of what was there originally). If you have multiple T4 slips, ensure that the total of box 18 does not exceed the amount on line 31200.