No macOS system backups were found on Time Machine

Hello,

Just downgraded my wife's Macbook Air from Catalina to Mojave. Erased the startup file and re-installed a "fresh" Mojave. Now when trying to enter the Time Machine I get the error "No macOS system backups were found on Time Machine". I know there are several years of information on the machine as I also use it for my Macbook Air. I wonder why I cannot suddenly find my wife's backups? The backups were there 4 weeks ago when I made the first attempt to do a backup from Time Machine. At that time I didn't erase the startup file so I ended up with two data drives that took up all memory on the computer.

Is there a way to read what is on the Time Machine so I can make sure the backups are still there? The memory available on the Time Machine still shows the same so I assume all the info is still intact. Appreciate the help!!