To review dependency errors in LabVIEW NXG, you can Open your project>>Project Files>>Right click in your project>>Capture Packages Dependencies
Here LabVIEW NXG will review all the files that you are using in this project and will report any dependency error of your project.
Based on the name of the missing VI, find the NI Package Manager package that includes the VI. Install the package to get the missing subVI.
If the missing subVI is one that you have created, manually add the subVI to the project. Once the subVI is in the project, your VI should be able to open without errors.