Check LookAt function in BP_TestActor, it should have runtime error. Maybe you deleted or renamed a variable in a struct?
If this actor doesn’t have runtime error, try refreshing all nodes: Go to EditorPreferences and check if you have any key bound to RefreshAllNode. If not, bind something comfortable (I usually use ctrl + R). Then go to test actor and press that key to refresh nodes. It should show the error or fix this error.
Thank you for your answer. I went to the Blueprint and I was able to correct the problem. I needed to compile it again and do the same for two others Blueprints.
Thank you again!
Have a great day !