Why can’t I save pdf from notes app under OneDrive?

It has become a bit cumbersome, but it can still be done.

Do as follows:

1) Go to the Note you want to save as a PDF.

2) Select (...) in the upper right corner

3) Select Send copy

4) Select Markup

5) Without changing anything, just press OK in the upper left corner

6) Select "Save File to .." and save it in the Files app.

Now the Note is located as a PDF file in the Files app, from there you can upload it to OneDrive.

Hope it helped you.