Talking about this problem with a coworker and reviewing @carlinjm's suggestion lead me to going into the Forms view and checking out line 10b of "Form 1040 or 1040-SR Worksheet." Looking up info about the $300 (or $150 depending on filing status) that is mentioned I found you can (must?) apparently enter one of the two values. Once I did so I was able to get out of the "invalid entry" loop. Apparently TuboTax doesn't have the information it needs to enter this value for you. I did a search on "u.s. federal income tax charitable contributions" and found several hits with relevant information.
