In this circumstance, if the user receives a message which states 'An error occurred while updating Football Manager 2021 (content file locked)' or 'Disk Write Error' then we would suggest uninstalling and reinstalling Steam.
Details on how to do this here - https://support.steampowered.com/kb_article.php?ref=9609-OBMP-2526
If you have a large number of other games installed via Steam, we'd suggesting backing up before completing the install. We'd suggest uninstalling Football Manager. Save games are kept safe during the uninstall process and can only ever be removed if they are done so manually via the user.
Details on how to backup Steam files can be found here - https://support.steampowered.com/kb_article.php?ref=8794-yphv-2033