First, check that your antivirus isn't scanning the TB profile folder (add the folder as an exception or exclusion in AV settings). Next, if the account is IMAP, try rebuilding the Inbox, by deleting the mbox and .msf index file:

http://kb.mozillazine.org/Compacting_folders#Real_fix

e.g. Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, open the ImapMail/imap.gmail.com folder that has Inbox and Inbox.msf.

When TB is restarted, the Inbox will download from the mail server. See if compacting now works.

Thank you for the answer, but as I said above, rebuilding the indexes did not help. My account is POP and I had no benefit by deleting the "global-messages-db.sqlite" file and rebuilding it.

By default, Microsoft Defender Antivirus runs in the background, scanning files and processes that you open or download looking for malware.

When you want to compact or download emails to eg: Inbox, that file gets opened. If opened the Anti-Virus will then control the file which may prevent anything being written, so you need to make the Thunderbird folder (and all files and folders it contains) exempt from this auto scanning.

If you use a different Anti-Virus eg: Norton etc, then the same applies.

Have you Exited Thunderbird and then accessed your Anti-Virus and set up the Thunderbird profile as exempt from scanning? This is the folder I'm talking about: C://Users/username/Appdata/Roaming/Thunderbird

Then restart Thunderbird, wait for any download checks to occur and then right click on Inbox and select 'Compact folder'. Whilst compact is in progress, do not select to open or select anything and do not exit Thunderbird. Just wait. Did you get the same error message?

What size is the Inbox mbox file in the profile name/Mail/gmail account folder here: C://Users/username/Appdata/Roaming/Thunderbird/Profiles/profile name/Mail/gmail mail account folder

Thank you for the answer, but as I said above, rebuilding the indexes did not help. My account is POP and I had no benefit by deleting the "global-messages-db.sqlite" file and rebuilding it.

Rebuilding the index, which is the same as deleting the .msf file or doing Repair Folder, is not sufficient. Deleting the mbox file will make TB automatically recreate it on restart with a new, uncorrupted Inbox. Rebuilding the global sqlite file has no impact on repairing a corrupted Inbox.

I thank all those who are participating to help me solve the problem. I was not able to solve the issue even by carrying out the following activities that were suggested by you. I inserted the exception of the TB folder in the Antivirus (integrated in windows10) of the path: "C:\Users\Riccardo\AppData\Roaming\Thunderbird" In addition to the antivirus integrated in Windows10 I also use the Watchdog (last release 4.3.102) anti-malware protection, but there is no possibility of excluding individual folders or files. I turned off completely and restarted the PC. I reloaded TB waiting for all the services to load, mail downloads and so on. My path is: C:\Users\Riccardo\AppData\Roaming\Thunderbird\Profiles\xx1qf56m.default\Mail Inbox size = 0 kb and Inbox.msf = 3 Kb

However, I also ask the following question: "If for so many years the compaction that I perform daily, has worked well without excluding the TB folder from the antivirus, why should I exclude it now?" Finally, I note that inside my TB folder there is no "mbox" file as Sfhowes reported to me in the last post, but only inbox and inbox.msf by deleting which I did not obtain any result. Thanks again to everyone. Riccardo

I would like to add that now it no longer gives the error message: it simply does not compact the folder, leaving a few MB of unused memory.