When all else fails, read the Instructions  (to Form 8615) --

Parent's return information unavailable. If a child can’t get the required information about their parent's tax return, the child (or the child's legal representative) can request the necessary information from the IRS.

How to request. After the end of the tax year, send a signed, written request for the information to the Internal Revenue Service Center where the parent's return will be filed. (The IRS can’t process a request received before the end of the tax year.)

https://www.irs.gov/instructions/i8615#en_US_2024_publink1000101054 
The request must contain all of the following.

• A statement that you are making the request to comply with section 1(g) of the Internal Revenue Code and that you have tried to get the information from the parent.
• Proof of the child's age (for example, a copy of the child's birth certificate).
• Evidence the child has more than $2,600 of unearned income (for example, a copy of the child's prior year tax return or copies of Forms 1099 for the current year).
• The name, address, social security number (SSN) (if known), and filing status (if known) of the parent whose information is to be shown on Form 8615.

A child's legal representative making the request should include a copy of their Power of Attorney, such as Form 2848, or proof of legal guardianship.