Did you make a backup anywhere in you work flow?
Lightroom tells you that it has detected that you are using a backup catalog. Since that is indeed what you do, there is no problem. You had sync enabled in the original catalog, and if you enable sync with this one, then all the images will sync again because you can only sync one catalog. That's why you get this warning.
You don't have to enable sync at all. First, check that this backup is indeed the newest one you've got and see if there is any work you need to redo. When you do want to enable sync, it's the little arrow behind the name plate (the arrow appears when you hover the cursor over it).
What EXACT version of LR are you using?
With older versions you can open older backups as they are just COPIES of the catalog file.
With LR 7 (Maybe LR 6/CC 2015 at some point) LR started compressing the backups of the catalog file making ZIP files and it first needs to be Un ZIPPED, decompressed, before you can open them.