The issue may be to do with:
1. Network abnormalities
Change the network and try again.
2. Insufficient storage space of Google Photos
Empty the cloud storage space with the following methods:
1) Go to Google Drive, delete invalid files and clean up the storage space.
2) Go to Google Photo > Library > Trash > Select picture > Permanently delete, release space.
3) Go to Google Photo > Avatar in the upper right corner > Account storage > Review and Delete to delete invalid files.
3. Expand Google storage space. Go to Desktop > Google Drive > More in the upper left corner > Buy storage. Choose your preferred package; it'll need paying for.
If the issue persists, contact our Customer Support for further assistance.