OK, wait a minute. I tried to import a raw file from Apple Photos, and it gave me the same error message that you got. There may be a problem here that is not your fault, and it might be related specifically to having Photoshop import from Apple Photos. Because if I have Photoshop import a raw file from the iOS Files app, it works fine.
For now, if the raw files you want to open in Photoshop are in Apple Photos, I have tested this workaround and it works.
Instead of having Photoshop open the raw file from Apple Photos, export it from Photos in original raw format to the Files app, and have Photoshop open it from the Files app instead.
If you don’t know how to do that, these are the steps:
1. In Apple Photos, Share the photo (use the Share icon).
2. In the iOS Share menu, tap Save to Files. Caution: Exporting any other way will probably convert it to a JPEG, so be sure you specifically tap Save to Files.
3. Choose a folder in the Files app, and tap Save.
4. Switch to Photoshop, and tap Import and Open.
5. Tap Files (not Photos).
6. Go to the folder where you exported the raw file from Apple Photos, and tap it.
The file should open in Camera Raw in iPad Photoshop.