python - why I get ! [remote rejected] master -> master (pre-receive hook declined) in dokku

Question

I need help. I'm trying to deploy my Python/Flask app to Dokku server but I get an error and I don't have any idea of how to fix it. this is the git remote:

dokku   dokku@apps.danieldiaz.me:portfolio (fetch)
dokku   dokku@apps.danieldiaz.me:portfolio (push)

the app is created on the server, but when I make a:

git push dokku

or:

git push dokku master

I get this error:

Enumerating objects: 5, done.
Counting objects: 100% (5/5), done.
Delta compression using up to 8 threads
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 292 bytes | 292.00 KiB/s, done.
Total 3 (delta 2), reused 0 (delta 0)
-----> Cleaning up...
-----> Building portfolio from herokuish
-----> Adding BUILD_ENV to build environment...
-----> Unable to select a buildpack
remote: 2021/01/28 22:49:58 exit status 1
remote: 2021/01/28 23:49:58 exit status 1
To apps.danieldiaz.me:portfolio
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'dokku@apps.danieldiaz.me:portfolio'

The domain apps.danieldiaz.me works, I can connect via ssh.

question from:https://stackoverflow.com/questions/65946478/why-i-get-remote-rejected-master-master-pre-receive-hook-declined-in-do