This happened to me when trying to compile/run a Flutter application on Windows.  It’s quite an irritating bug and searching online found a 3-year old StackOverflow question about it.

There were several possible reasons for it.

• Files were protected. (i.e. needed unblocking).
• File path contained foreign characters.

I tried looking for these with no success. However I did find a font file that I’d added in the pubspec.yaml that had a | in the middle of the name. D’oh!

I followed it up with these commands in a terminal

flutter clean

flutter pub <span class="hljs-keyword">get</span>

And it worked.