@kersing You are correct, I was missing a few fields. I have now matched the API calls that are made on the web console. This has, however, not solved the problem.
Inspecting the data returned when trying to fetch a device, I found that the AppKey is missing, though this is added in exactly the same way as when creating a device through the console.
I’ll look more into this and hopefully find a solution, which I can share here in case someone else experiences a similar problem-if anyone has already experienced this and found a solution, feel free to share.
I was writing the details as I had the exact same issue as @tobiasfg + some more (couldn’t manually specify the app-key).
The solution was to move from ttn-lw-cli v3.19.1 to v3.20.0 and everything started working, where the same command with 3.19.1 would not (either because attempting to set the app-key manually and that reported an error, or successfully creating the end-device but the web console would report an error)
As this thread was actually about a console error that tells you what to do to fix it which happens often enough and largely fixes itself but if you do the instructions, does fix itself, and is not related to the CLI, closing before we accumulate any more red-herrings.