I went ahead and reformatted your description a bit:

Dear support team, When creating a publication of a deb/apt repository in structured mode, I get the following error :

  File "/usr/local/lib/pulp/lib64/python3.6/site-packages/rq/worker.py", line 886, in perform_job
  rv = job.perform
  File "/usr/local/lib/pulp/lib64/python3.6/site-packages/rq/job.py", line 664, in perform
  self._result = self._execute()
  File "/usr/local/lib/pulp/lib64/python3.6/site-packages/rq/job.py", line 670, in _execute
  return self.func(*self.args, **self.kwargs)
  File "/usr/local/lib/pulp/lib64/python3.6/site-packages/pulp_deb/app/tasks/publishing.py", line 137, in publish
  prc.package
  File "/usr/local/lib/pulp/lib64/python3.6/site-packages/pulp_deb/app/tasks/publishing.py", line 174, in add_package
  package_serializer.to822(self.name).dump(self.package_index_files[package.architecture][0])
  KeyError: 'all'

Commands used to create the repository, remote and publication :

http post :24817/pulp/api/v3/repositories/deb/apt/ name='deb-test3'
http post :24817/pulp/api/v3/remotes/deb/apt/ name='deb-test3' url='http://ftp.ch.debian.org/debian/' policy='on_demand' distributions="buster jessie" architectures="amd64" components='main contrib'
http post :24817/pulp/api/v3/repositories/deb/apt/52bdc8ed-fc59-4825-9c75-1240d00e8a30/sync/ remote=/pulp/api/v3/remotes/deb/apt/8f040ed3-8b25-4dcf-ba7f-be10cc55a813/
http post :24817/pulp/api/v3/publications/deb/apt/ repository=/pulp/api/v3/repositories/deb/apt/52bdc8ed-fc59-4825-9c75-1240d00e8a30/ structured=true

Pulp component versions (obtained via 'pip list | grep pulp'):

• pulp-deb 2.3.0b1
• pulp-rpm 3.3.1
• pulpcore 3.3.1

pulpcore + pulp-rpm have been installed using the Ansible installer. pulp-deb has been installed using this procedure:

su - pulp
export PULP_SETTINGS=/etc/pulp/settings.py
source /usr/local/lib/pulp/bin/activate
/usr/local/lib/pulp/bin/pip install pulp-deb
pulpcore-manager migrate --noinput

Looks to me like the structured publisher is trying to place "architecture=all" type packages into their own non existent Packages file.

This is clearly a bug that needs to be fixed.

You could try using "simple=true" instead of "structured=true" until this is fixed.

The following PR fixes this issue: https://pulp.plan.io/issues/6787

Additionally we should also update the test fixtures to cover this case.