The Unlock PDF page's password input is marked required, so the form will not submit without a typed password value, even for a file that has no open password.
Adobe's own PDF-unlocking guide states that removing a permissions/restrictions password still requires typing that password into the tool, rather than stripping it without any password. How to remove a PDF password in 3 easy steps | Adobe Acrobat
Unlock PDFRemove PDF Print Restriction Without Password
The Unlock PDF page's confirm-password input is also marked required, so both password fields must be filled before the form can be submitted.
The Unlock PDF tool always passes whatever value was typed into the password field to qpdf's --password flag, regardless of whether the source PDF has an open password or only permission restrictions.
Enter the owner password into the password field.
Enter the same owner password into the confirm-password field.
Handling Permission-Only PDF Files
Upload the PDF file to the Unlock PDF page.
Submit the form itself only after confirming you actually already know the document owner's own password.
Search for a separate PDF editor to remove restrictions without a known password.
Verifying Print Access After Unlocking
Download the processed PDF file.
Open that downloaded file afterward in any standard PDF viewer application already installed.
Select the print option to test access.
Continue with Unlock PDF requires known password or Unlock PDF wrong password retry.
Frequently Asked Questions about Remove PDF Print Restriction Without Password
How do I submit the Unlock PDF form for a file without an open password?
Type the owner password into both required fields before submitting.
Where do I find the owner password for a restricted PDF?
Search your inbox for the original email that shared the file.
How do I print a PDF that blocks printing?
Use a PDF editor that allows removing permission restrictions.
What should I do if I do not know the owner password?
Contact the document creator to request the owner password.