Update to my previous post (see below):
I tried the hearing test again on my iPhone 12 Pro using my over-ear aviation headset with gel ear seals covering the ears with the airpods pro - and it worked!
previous post:
Same problem with me - "Unable to Classify" because of background noise. I tried the hearing test on my iPhone 12 Pro with iOS 18.1 more than 15 times, but always failed during the test for the left ear, the test canceled after about 60-80 %. I also tried on an iPhone 12 mini with iOS 18.1 and the hearing test worked 3 out of 6 times. I did the tests in a very quiet room, there was absolutely no noise. I took the data from the successful tests on the iPhone 12 mini and transferred them manually to the Health app on my iPhone 12 Pro so the AirPods can now use that audiogram for adjustments.