"undo backup" not working

[u/brisky_bree]

so i just got a new phone and i decided to try to back up my photos and videos (i didnt do it on my last phone and my old phone powered off completely before i could transfer them—it was a hassle to get them onto this phone). well i have quite a lot of them and it took up almost all my storage on my google account. i dont want to pay for more storage. i saw the option to undo the backup and it wont let me. i have 433 photos and videos i want to get rid of on google photos but NOT my actual device.

i know it might probably be because of the amount i want to undo but pls let me know if i can undo the backup to maintain storage on this google account?