Can’t delete an old phone backup

I’ve recently had a new iPhone ( Feb) . Today I am having a new iPad so doing a clean up of my old iPad so I can just transfer everything over to the new one . I have noticed that there is an old backup from my old phone that I did when I got my new one. I have tried to delete it but it keeps saying I can’t delete it . I have tried turning off iCloud back up and trying again . I have tried disabling and deleting my photos as they seemed to take up the most space . Everything i can find on line about this problem I have tried but nothing has helped . This backup is over 8 gig so would love to delete it to free up some space . Thanks in advance