Hey there!
Make sure of the following:
Your app and the extension targets have the same
minimum deployment target
version
Both targets contain necessary entitlements (depending on your NE type)
<key>com.apple.developer.networking.networkextension</key>
<array>
<string>packet-tunnel-provider</string>
</array>
Your network extension target bundle id just extends the main app bundle id
Main app: com.your.main.app
Extension: com.your.main.app.extension
Your main apps target contains YourNETarget.appex in Framework, Libraries, and Embeded Content section