Some customers have run into an issue where InDesign files are consistently opened as read-only even though permissions appear to be correct and no one else is using the file.

If you are using Mac (possibly limited to Big Sur) and Sophos, turning off CryptoGuard (ransomware detection) may help:

If you cannot open the InDesign file at all, there may be an InDesign lock file that's preventing the file from being opened. In this case, look for the .idlk file in the same folder. This is the InDesign lock file used by the appliRemoving the .idlk lock file may allow accessing the file again.