airport will not recognise existing backup disc
airport has not run a back up for 3 weeks as it will not recognise the airport disc.
macOS High Sierra (10.13.3)
You need to provide a lot more information than that.
Most important to understand the issue is High Sierra.. which has many problems with Networking.
It is Time Machine that does backups.. the airport Time Capsule.. TC from now on.. is merely the dumb as a board target device.
Tell us the whole network setup.. or link us to previous posts..
What have you done to troubleshoot?
Most important thing you can try is a restart.
Turn off everything.. restart in correct order.
1. Modem.. wait 2min
2. Time Capsule.. wait 2min
3. Computer.
Check that TC is visible in Airport Utility and under shared in the Finder sidebar.
If Time Machine cannot find it still.. I highly recommend you download Carbon Copy Cloner and give it a try.
Honestly trying to fix issues with TM will end up back here again.. it is simply unreliable.