From your stack trace it seems like you’re just missing the supportedPlatforms property in the plugin configuration.
When it comes to Appodeal there has been a lot of problems lately, with outdated SDKs etcetera, at least for myself. I just got it to work when building for iOS 14 thanks to @agramonte who has supplied us with some updated plugins at the marketplace. See this topic for more info: Is 14.0 BETA ok to use for release build?
Just post in this topic if it doesn’t work out for you and I’ll try to help.