In reply to Joe.

I too was having the same problem, and finally got it to work.

I kept mine simple ocrmypdf "$1" "$1"

The trick though was the quotation marks. Regardless if I typed it in or copied pasted from a browser it resulted in "File not found"

The work around was I put Hazel in debug mode:
1) In the menu, select “Hazel→Preferences”
2) Hold down the option key.
3) You should see a new pane called “Debug”. Select it. You can release the option key.
4)Click the “Enable Debug Mode” checkbox.

and ran the rule once. I took the quotations used in the debugger and replaced the quotations in the script. (ocrmypdf "$1" "$1")

On my screen the ocrmypdf part is white; " " are red; and $1 is green if that helps too.

I presume the quotations in terminal might work as well? To see if it's ocrmypdf or hazel causing the issue, I recommend in terminal typing
"ocrmypdf" a space and drag the file into terminal, once for the input and once more to use the same name as the output (or type a name "OCR.pdf")

Good Luck.

TL;DR:

Copied and pasted quotes in Hazel debugger to replace the quotations on the script (ocrmypdf "$1" "$1")