@BR31, Welcome to HP Support Community!
Normally the printer will exit single-cartridge mode when it detects the second ink cartridge. It seems that the printer is currently not detecting both ink cartridges.
Try this:
Remove the ink cartridges from the printer.
With the printer turned on, disconnect the power cord from the rear of the printer.
Unplug the power cord from the wall outlet.
Wait at least 60 seconds.
Plug the power cord back into the wall outlet.
Reconnect the power cord to the rear of the printer.
Turn on the printer, if it does not automatically turn on.
The printer might go through a warm-up period. The printer lights might flash, and the carriage might move.
Wait until the warm-up period finishes and your printer is idle and silent before you proceed.
If you are still having a problem, try this document- HP Printer Does not Print and Error Indicates Faulty Ink Cartridge
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TEJ1602
I am an HP Employee