Cannot create backup copies

I have issues with Time Machine on iMac 27", macOS. The thing is, I cannot create copies on backup and nothing I do worked. It shows as if files are uploaded but by the end, no files are actually stored online. What might be the cause as I did not do any updates on the system and it worked well before?

Hey there!
The problem could be that your storage might be full, no space for new ones to safe