I’d like to make assets be unable to be saved and preferably add a message explaining the missing fields in the KMC. For example, if a user is uploading assets and fails to include a critical field, we’d need to stop them from adding the item to the library until it is filled out. Is this possible?
I assume you are talking about the custom metadata fields. There no way to force filling these when creating an entry from KMC, however, if your users upload entries from an external system using the API, you can obviously do that during the upload submission stage.
You can also set moderation in KMC so if your users are uploading from Kaltura Media Space, for example, it will not become public until someone who has the right privileges goes onto KMC and approves them.