Most likely you are using some external app store modules with java actions in them. As Mendix 9 is still in beta stage, many of the app store modules are not upgraded/compatible for Mendix 9 yet.
At this stage if you are using modules from app store, best is to stay with latest stable release Mendix 8.17.0
Likely, if you press ‘View error details’ you will see the little arrow-up ^ pointing to Core.Execution of one of the modules that are not yet 9.0.2 compatible. You can try and re-import that particular module from the appstore, if there is a newer release than yours, but most likely you will have to patiently wait for Mendix 9.1.0