Everyone has experienced problems while they are about to backup pictures. Google Photos is an efficient app; however, it can get stuck while preparing backup. There is nothing to worry about as such an issue can be resolved easily. Several factors such as slow internet connection or insufficient memory can cause the issue. It is also because of various temporary bugs in the Photos app. If you wish to fix this issue, then you can try the given solutions to backup the pictures.
Please read the originial post here: https://camsoffice.com/blog/how-to-fix-google-photos-is-stuck-on-preparing-backup/