I had the same issue - it turned out that the charities I had donated to in 2019 came over so I had to enter a value greater than 300 in one of them. Once I did that, then the validation started working. To get there, go to Federal > Deductions & Credits, then expand Charitable Donations and click Start/Revisit next to "Donations to Charity in 2020". If it tries to encourage you not to fill it out, just click the option to enter it anyway. Then you can add an amount.
