I’ve got a project that was originally created in Eagle which I’ve now managed to get imported in KiCad (6.0.5)
Schematics and PCB layout all look ok, except whenever I now open the PCB Editor I get a error items found on undefined layers. And get a option to delete or rescue them. I have tried deleting and rescue options which don’t seem to do anything. The PCB opens after clicking delete/rescue but when I close and re-open the PCB I get the same error.
First you should have a look at what sort of items are on that layer, and do something with them if they are of any importance to you.
Does the layer show up in: PCB Editor / File / Board Setup / Board Stackup / Board Editor Layers or in the Appearance Manager on the right side of the screen?
As a last resort, you can open the PCB file in a text editor, search for the name of the layer and delete, rename or move S-Expressing belonging to that part.