“We couldn’t register this document” error in Word or “The file could not be accessed” error in Excel when opening file from SharePoint

You may receive the following error when opening a Word document from SharePoint:

We couldn't register this document. You won't be able to create links from other documents to this one.

Or you may receive the following error when opening an Excel workbook from SharePoint:

The file could not be accessed.

These two errors can happen if the file has a long file path. Per the following document, SharePoint on-premise has a 260 character limitation in the full file path, which includes the path and the file name:

https://technet.microsoft.com/en-sg/library/ff919564(v=office.14).aspx

The limitation in SharePoint Online for the file path is 400 characters:

https://support.office.com/en-us/article/SharePoint-Online-limits-8f34ff47-b749-408b-abc0-b605e1f6d498

To fix the error, reduce length of the file name so that the full file path is below the character limit.