nbgrader autograde cannot find file even though notebook exists

[TianSLuan]

Operating system MAC

Expected behavior

I am trying to use nbgrader autograde in the command line to grade assignments
The submitted assignments are stored in

submitted/s1/ps1/ps1.ipynb and
submitted/s2/ps1/ps1.ipynb

I am hoping to get the grades for s1 and s2.

Actual behavior

When I run the line nbgrader autograde ps1 in the course directory, I keep getting
[Autograde | ERROR] No notebooks were matched by '.../submitted/*/ps1' (the ... just omits the root directory)
What am I doing wrong and how can I fix this?