mom here. where do i go to make the box on my daughters return to say that she is a dependent of mine. I forgot to mark that box and now my return has been rejected.

You are amending her return using a Form 1040X?   On her return go to MY INFO and click her name.   Go through the screens to the question that asks if she can be claimed as someone else's dependent and answer YES to that.   Did she already e-file?   You do not have to wait for her amended return to be processed but since this mistake occurred, you have two options---print, sign and mail your own return that claims her as your dependent.

Or see if you can get an IP PIN for her and use it on your return to e-file.

GET IP pin in order to e-file if your dependent’s SSN was claimed on another return

https://www.irs.gov/newsroom/irs-takes-steps-to-help-prevent-refund-delays-by-accepting-duplicate-dependent-returns-with-an-ip-pin-for-2025-filing-season-taxpayers-encouraged-to-sign-up-soon-for-ip-pin-online-account

https://www.irs.gov/identity-theft-fraud-scams/get-an-identity-protection-pin