@duncfair, What are you using as your trigger? I was using a mobile button, just to test. Didnt work. Samee issue as you. I did this with a Powerapps trigger, and it worked. HOWEVER... I did have to re-select the connector during my test run. Its like it didnt know it was really set or something. Despite it being set in edit mode. Take a look at that.
I'm getting this too. I believe it needs escalated. I can pick a file explicitly in the action, and then it cant find it when run. Explicit selection rules out a connection problem, or it would not show you file structure. I also get this error from a "List files in root foler" action, which has no input. That rules out a syntax problem. I am using the same connector as that from another flow where it is working. My Flow is brand new. Not copied. So, command syntax is not the issue, and connector is not the issue. It must be a deeper Microsoft problem.
MS... please help!