How can I unlock the file below on Mint 19?
This is a LibreOffice document I had opened when my PC crashed.
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Linux has no unified locking mechanism on files. There are various types of advisory locking mechanisms but they don't work on all file systems. This means applications usually use their own mechanism. The most common trick is to create a hidden "lock" file next to the original. If the hidden "lock" file exists, the application will refuse to open the file.
See here for LibreOffice: https://ask.libreoffice.org/en/question/110863/files-locked-for-editing/
Ususally you would remove the lock by closing the application, but with lock files, the application can crash without first removing the file. If this happens you have to remove the file manually.
I suggest you try to view hidden files in the same directory and look for a file named something like:
.~lock.1 Mint 2019.odt#