My fed tax return could not be filed in April bc of a mistake which wouldn't let me to claim my adult child. I filed an extension. I am ready to file now, how to proceed?

Since your daughter filed a federal tax return indicating she was not a dependent, the IRS will not let you e-file your 2023 tax return if you are claiming her as a dependent on your tax return since they have already received a tax return with her Social Security number.

You will have to mail your 2023 tax return.  You need to print, sign, date and mail the 2023 return to the IRS.  You need to include all forms W-2 and 1099 with the return which have federal taxes withheld.

Go to this IRS website for the address to mail, by state. with or without a tax payment - https://www.irs.gov/filing/where-to-file-paper-tax-returns-with-or-without-a-payment

FYI - - you didn’t have to wait to file since you need to mail in your return.  Do you have a refund or tax due?  If you have a tax due it still needed to be paid on time by April 15 even if you got an extension to file.  

She needs to Amend her return, did she?  Even if she amended you still have to mail yours.   Amending won’t release her ssn.  Once a ssn has been filed you can’t efile it again.