Why does the Printer Offline error occur?

The error appears when Windows 10 sees that the printer is unavailable.
Unfortunately, most of the time, it can't tell if the printer is really offline or if it has connectivity problems or printing errors. They might occur when:

The connection between the computer and the printer is slow / unresponsive
The printer has encountered an internal error
There are multiple unfinished print jobs in the queue

Source:  printer is offline