You have a folder with many levels of subfolders which were created or moved in OneDrive and synced without issues to the cloud.

However, when trying to open certain .xls files in Excel 2013 or .xlsx files in Office for Mac stored in deeply nested subfolders, an error message comes up: "Sorry, we couldn't find <filepath>. Is it possible it was moved, renamed or deleted?"

When trying to open that same file using Excel 2016/2019 on Windows, the error message states:

That same file can be opened in Excel Online (the web version of Excel) without any issues.

This is is being caused by the file path being too long.

Excel seems like it is currently unable to open any files that exceed 256 characters in their file path. You have to reduce the number of subfolders in the file path [i.e. "Move the file up"], or shorten the file name.