You might consider to delete all of your Restore points (old ones) and run a software like CCleaner to get rid of all the junk in your computer.
Restore points can take up to several GBs even 50 GBs worth of Disc space without showing up.
You can google how to clean out your Restore point folder.
Also go to the hidden folder in your User account called APPDATA and go to Local folder and then click on TEMP folder. Delete everything there. Some items you won't be able to delete so just click on "skip".
Also go to Windows settings - System - Storage and see how much space is show as being free. There is will show you all the programs you have installed and the amount of space each uses.
I had a similar issue with my wife's computer. I tried to do a System backup but kept getting that the disc I was going make the copy of the System Backup didn't have enough space. It had over 150 GB of free space. Then I realized the size of the System Backup was almost 700 GB large.
So I went to Windows Settings - storage and deleted a couple of very old large program she had installed. Still it made little dent in the size of the backup.
So I went to MyPC and clicked on the Windows Drive. I went to each folder in the C:\ Windows drive and clicked on "Properties"to see how large each folder was. Found that the User Folder had over 600 GB of disc space used. Finally, through process of elimination, found that a program that my wife uses to convert Audio/Video files had made a backup copy of all the Audio/Videos she had made in the past in the program's folder.
Deleted all the backup Audio/Videos and now the C: Windows drive went from being over 700 GB large to less than 52GB large. Now I easily was able to do a System Backup that took up less than 50 GB of space.
