On page 1 of T1, there is a field for spouse net/taxable income. Due to space limitation, only the spouse's net income is displaying. When you click mouse into the field or try to type a number into that field, a supporting field window will pop up, like the following:
Version 2020 and later (Click to view)
Version 2019 and prior (Click to view)
There are both spouse net income and taxable income in this window. Make sure both spouse income fields are entered, and taxable income should not be more than net income.