I didn't input my kid's college tuition expense when I did my taxes last year for tax year '20. Is it possible to credit those expenses against my tax liability for '21?

Qualified education expenses paid in 2020 have to be entered on the 2020 tax return.

You will need to amend the 2020 tax return to add the education expenses paid.

See this TurboTax support FAQ for amending a tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-2020-return/L3XdA1Pw8_US_en_US