It could be a program limitation. If you are in Forms mode, go to 'Step by Step' mode. Can you click into another area, like WAGES & INCOME, then click back into DEDUCTIONS & CREDITS and Edit CHARITABLE DONATIONS?
Have you tried signing out/in again and going back to that section?
Did you have mileage for more than 10 different charities? You can select 'Various' for dates of mileage for each charity and then just enter Total Mileage for each charity.
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