There was a bug, fixed in 1.12.2, preventing Ruins from recognizing minecraft:air as a legitimate block ID. Since you're running a version without that fix, you'll need to replace all occurrences of minecraft:air in your templates with simply air. That should eliminate the errors.
Note for all other blocks, the minecraft: domain works fine, so you only need to remove it from air blocks. For example, stone and minecraft:stone are both valid and interchangeable. Just not air.
Ah, gotcha. Can do!
Thank you very much!