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
Remove and reinstall your printer
Another way you can fix an offline printer is to remove the printer from your PC or laptop and reinstall it. To remove your printer, simply open the ‘devices and printers’ in your computer’s control panel. Right click the model you’d like to remove and select ‘remove’
Source: printer offline