How do I increase size of encrypted folder

Catalina 10.15.7

Theoretically I should use hdiutil resize -size 1024m [path] [Enter], at which point Terminal should request the password of the encrypted folder. However when I try this Terminal responds, "resize: files. Resource temporarily unavailable (35).

What am I doing wrong, or is there another way?

I use to use Disc Utility, but now when I attempt to use Disc Utility, I highlight the encrypted folder, in Disc Utility, then under "Images" click "Resize..." Disc Utility responds, "The selected disc image can't be resized. Resource temporarily unavailable."