Sounds like the master sheets you are trying to delete are in use. Change the sort method in the navigator to by master and you can see which layouts are still using the old masters and change them. Then you can delete the unwanted master sheets.
Also if a layout haas a master applied - that one cant be deleted either
Cheers John
John Hyland : ARINA : www.arina.biz
User ver 4 to 12 - Jumped to v22 - so many options and settings!!!
OSX 10.15.6 [Catalina] : Archicad 22 : 15" MacBook Pro 2019 [/size]