That is not a normal Windows 11 message. Because the computer is a refurbished OptiPlex, the most likely explanation is that it still has a startup program, scheduled task, or management agent left behind by the previous company or refurbisher. It is trying to contact their deployment server, which is no longer available.
After you received this computer, did you do a full reset? or clean install?
Can you hold down Ctrl+Alt+Del, launch Task Manager, then go to Startup tab, and sort by status column, so enabled ones are on the top. Then share a screenshot pls.
Next, hold down Win+R keys, type in shell:startup and press enter. It should launch a folder, take a screenshot to show me what's in the folder pls.
Also, Can you please type cmd in the search on the taskbar and hit enter to launch command prompt, then run the following command:
slmgr /dlv
Then share a screenshot please. Thanks!