Why is "Why couldn't she get tickets" correct instead of "Why couldn't she got tickets"?
Why is it get in the present instead of got in the past tense?
All modal verbs (can may must shall will could might should would) are followed by the bare infinitive For most verbs the bare infinitive is the same as the first person present tense.
In English the tense is indicated in the modal verb, and the main verb doesn't have tense (it is infinitive)
So "couldn't got" is a grammar error and "couldn't get" is correct.
Note that "couldn't was" and "couldn't am" are both incorrect grammar. The infinitive form is "be". "Couldn't be" is correct.