Form 8615, I don't know my parent's taxable income? problem?

Ask them.   The IRS allows you to file for an extension  to file until Oct 15, 2021 to give you additional time to get that information.

The 8615 instructions says this:

https://www.irs.gov/pub/irs-prior/i8615--2020.pdf

Incomplete Information for Filing
If the parent's taxable income, filing status, or net unearned income
of the parent's other children isn't known by the due date of the
child’s return, reasonable estimates can be used. Enter “Estimated”
next to the appropriate line(s) of Form 8615. When the correct
information is available, file Form 1040-X, Amended U.S. Individual
Income Tax Return.
Instead of using estimates, the child can get an automatic
6-month extension of time to file. For details, see Form 4868,
Application for Automatic Extension of Time To File U.S. Individual
Income Tax Return.