if the employer paid tuition was a nontaxable fringe benefit, ie you didn't pay taxes on it, you get no tax deduction for repayment.
if it was included in compensation, you do not amend 2019 since the repayment was not until 2020.
for 2020 your choice:
1) deduct the repayment on Schedule A in the section for other itemized deductions. if you can't itemize you get no benefit under this method
2) recalculate the tax for 2019 as if the repayments had been made in that year. the difference between the original taxes and revised taxes is a credit to claim in 2020 on schedule 3 line 12d - describe as "IRC 1341"
