I made over $300 in donations. It said I could enter up to 300 so I did, then I got a message "cash charitable is too large" I lowered to 100 and still says "too large"

you can not take a page 1 deduction for charity if you itemize deductions

if you don't itemize you must use the 1040 worksheet to enter an amount on 10a and then an amount not to exceed the larger of 10a or $300 on 10b