If after you do a full install update for WHMCS and you are receiving:

"Update in progress, please log off and try back later. Thank you"

You will need to complete the upgrade by:

Browsing to your whmcs home path /install

Once complete, remove the install directory.

I have not done a full update in years, and the auto-update from the control panel continued to fail with no reason, just that the upgrade failed and check the min requirements in which I did check and all were OK.

Then I did remember that I was needing to route to a URL to complete the install but since the summary full install "only" mentions to "Visit your WHMCS installation VS mentioning "install directory" - my 1st reaction was my normal path to where WHMCS lives. (Resolved above by way of the install directory)

"Visit your WHMCS installation and you should then be automatically redirected to the update process. If you see a message saying WHMCS is not installed, this indicates you have overwritten the configuration.php file for your installation. Stop and restore that file from your backup before continuing."

https://docs.whmcs.com/Updating#For_a_Full_Release_Version

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