It is legal: Jupyter notebook integration for the Wolfram Engine is maintained by WRI on the main Wolfram GitHub.
Maybe try importing a notebook with the "Import" function? "Import" works for me on the wolfram script CLI.
The "NotebookOpen" function does not seem to work for me on the CLI:
NotebookOpen["test.nb"]
gives me
FrontEndObject::notavail:
A front end is not available; certain operations require a front end.
Out[1]= $Failed
