As mentioned in step 10 of the directions here: Corrupt or Otherwise Broken Audacity Project Recovery, the message “unable to open database file”, [means] you have simply mistyped the file name or the .aup3 file name extension.
Note: Windows sometimes hides file extensions. To make them visible, go to File Explorer, and click on the View Tab. On Windows 10 in the three checkboxes, select “File name extensions”; On Windows 11, select “Show”, then “File name extensions”.