My federal taxes keep getting rejected and it’s saying my SSN is already used for a dependent and idk how to change my thing from single to dependent from someone else

Do you mean that someone else has claimed you as a dependent and you were trying to file and say no one else can claim you?   Which is correct?   CAN you be claimed as someone else's dependent?  Can you be claimed by a parent or other family member?   Are you a student?   Please explain so we can be more helpful.

To change from single to dependent of another, you need to go back to the "My Info" section in TurboTax and check the box to indicate that "Another taxpayer can claim me as a dependent on their tax return" and then answer "Yes" to Will this person claim you on their 2024 tax return?  Once you have done this, you should be able to e-file successfully.