Open the project suite (.pjs file),the pop-up window reports an error

I can suggest several starting points:

1) uninstall/reinstall

2) save/close the project, open a File Explorer and delete the files within your project's Script folder (be sure to copy them to a separate folder if you need them), then attempt to re-launch and hopefully re-import any existing script files you need. If it's a new project and you don't have any existing scripts yet, just delete the entire Script folder.

3) In the event you are using a version control system like Git, you may need to simply remove any hanging 'untracked' items. To clean you local git workspace you can use something like this from cmd:

*** removes all untracked files, packages, configs, artifacts, logs, etc ***
cd C:\<path to your TC project folder>
git clean -d --force

It looks like one or more scripts may have been deleted in the Windows directory instead of from inside TestComplete. I think what I did the last time I saw this was to add a script inside TestComplete and then delete it there, and then maybe delete the folder too.  If there are other scripts there, you can Remove them inside TestComplete before you delete the folder.  Remove will leave them in the Windows directory so you can add them again later if you need them.

I'm able to reproduce the error,

For example, if Unit1.js is missing, then modify Script.tcScript and remove the offending reference to the missing file, then reload the project. Or add the missing file back to the directory path as indicated, then reload the project.