can't get over "You do not have permission to modify this app"

I tried:

rm ~/.appcfg_oauth2_tokens

Whole process with page authentication finish successfully for both accounts (owner role) with same result

appcfg.py update --no_cookies

Same result

appcfg.py update --noauth_local_webserver

Token is displayed, accepted in terminal, process continues towards same result.

I also tried to add another google account as owner, with same result. When I mistype application name, error is different, so I guess this is not a problem. Goapp deploy and appcfg seems to be same tool basically, both have same results in the end.

Any ideas?

Try checking the appengine-web file at /webapp/WEB-INF sounds like you have a reserved application name or application+version

Edit: For further consultation I am adding the conclusion of the thread.

Deploying from the GCP console was a valid workaround to the issue.

https://cloud.google.com/sdk/gcloud/reference/app/create

https://cloud.google.com/appengine/docs/python/tools/uploadinganapp

Hint from Nilo_DS was pointing in right direction. Using gcloud commands, I was able to upload app correctly. Still not sure what is going on when using goapp command, but this workaround is good enough for me.
Commands I used were

gcloud init
gcloud beta app create
gcloud deploy