After migrating a project from Windows Legacy compatibility to Windows compatibility, some activities are missing. This message appears instead of the activity "This activity is either missing or could not be loaded properly".
If dependency issues cause unresolved activities after the conversion, a message displayed at the top of each such activity indicates that it is missing or it could not be loaded. The following information is available for each one: display name, activity namespace, activity properties, and values. Click Replace to find an alternative and replace the missing or invalid activity. For more information, see Missing or Invalid Activities.
For the official activities, offer support, and most of them will have packages that work in both compatibilities, but for unofficial / custom activities packs like ones from the marketplace, address the question if they have/will upgrade their activity pack for Windows compatibility projects directly to the developers who made the activity pack. Go to our forum and get more info there (alternative activities use or time frame for the update of the activity pack).