@tim28 I’ve just done some playing in Zapier, and the only way I can replicate the same error code that you are getting is by making a mistake in the “Authorization: Bearer” section.
One thing I noticed is that it’s easy to end up with a superfluous space between “Bearer” and your API key. I found that adding a space between the two didn’t actually move the text at all - Bearer Key looks identical to BearerKey. It will not work if there is a space between the two. Give that a go and see if that works for you.