OneDrive on Mac software update

Hi

I have what seems like quite a complex problem with OneDrive for Mac here. 

My MacBooks is currently on iOS 10.14. OneDrive for Mac (I think) is no longer supported on this version of the operating system. I do not know this for certain, but it would explain why the OneDrive application is completely unresponsive.

The obvious solve to this would be to do a software update. However, I cannot. I have 128GB of storage on my MacBook, the overwhelming majority of which is taken up by files which I have backed up to OneDrive. Unfortunately, I cannot access these files, either to delete or to move to a hardrive. As a result I do not have enough storage to install my software update.

I thought I had troubleshooted this problem by adding the inaccessible (OneDrive-backed-up) files to an enclosing folder, moving that folder to trash, and then permenantly deleting. But, when I view the storage manager, the available space on my disk has not decreased - it has just shifted from the "Documents" allocation to the "System" allocation (see attached pic).

I think, therefore, that while the files I tried to delete are still on my disk somewhere, albeit now invisible.

Would appreciate some assistance in resolving this issue. 

Many thanks

Freddie