You must have entered some data into the TurboTax program that indicated you meet the filing requirements for Form 8615.
It's not necessary that you be claimed on someone else’s return for the form to be required.
According to the IRS's Instructions for Form 8615,
Form 8615 must be filed for any child who meets all of the following conditions.
1. The child had more than $2,200 of unearned income.
2. The child is required to file a tax return.
3. The child either:
a. Was under age 18 at the end of 2020,
b. Was age 18 at the end of 2020 and didn’t have earned income that was more than half of the child's support, or
c. Was a full-time student at least age 19 and under age 24 at the end of 2020 and didn’t have earned income that was more than half of the child's support. (Earned income is defined later. Support is defined below.)
4. At least one of the child's parents was alive at the end of 2020.
5. The child doesn’t file a joint return for 2020.
For these rules, the term “child” includes a legally adopted child and a stepchild. These rules apply whether or not the child is a dependent. These rules don’t apply if neither of the child’s parents were living at the end of the year.
