The error is stating that you can’t call ApplyWebConfigModifications() on $app.Parent. In this case, it is the Parent property of $app that is null.
SPWebApplication’s Parent property is of type SPPersistedObject which doesn’t have an ApplyWebConfigModifications method and is most likely null in this case as SPWebApplications aren’t hierarchical/nested.
Try to restart following service in services.msc:
Restart SharePoint Server search 15
Restart SharePoint Timer Service
If the issue still exists, delete the existing SSA and recreate the search service application to check if crawl works well.
Disable-SPFeature -Id 69cc9662-d373-47fc-9449-f18d11ff732c
More details check this blog!