Flask app deployed to GCP Run through bitbucket

Question

I am struggling to get my dockerised flask app to a GCP run instance through bitbucket pipelines.

Here is my bitbucket-pipeline.yml:

image: python:3.9

pipelines:
  default:
    - parallel:
        - step:
            name: Build and test
            caches:
              - pip
            script:
              - pip install -r requirements.txt
              - pytest
        - step:
            name: Linter
            script:
              - pip install flake8
              - flake8 . --extend-exclude=dist,build --show-source --statistics

  branches:
    develop:
      - parallel:
          - step:
              name: Build and test
              caches:
                - pip
              script:
                - pip install -r requirements.txt
                - pytest
          - step:
              name: Linter
              script:
                - pip install flake8
                - flake8 . --extend-exclude=dist,build --show-source --statistics
      - step:
          name: Deploy to Development
          deployment: Development
          image: google/cloud-sdk:latest
          caches:
            - docker
          script:
            - echo ${KEY_FILE_AUTH} | base64 --decode --ignore-garbage > /tmp/gcloud-api.json
            - gcloud auth activate-service-account --key-file /tmp/gcloud-api.json
            - gcloud config set project PROJECT
            - gcloud builds submit --tag eu.gcr.io/PROJECT/APP
            - gcloud beta run deploy APP --image eu.gcr.io/PROJECT/APP --platform managed --region europe-west2 --allow-unauthenticated

options:
  docker: true

My Dockerfile:

# syntax=docker/dockerfile:1

FROM python:3.9

ENV APP_HOME /app
WORKDIR $APP_HOME
COPY . .

COPY requirements.txt requirements.txt
RUN pip3 install -r requirements.txt

COPY . .

CMD exec gunicorn --bind :$PORT --workers 1 --threads 8 app:app

and, the error from bitbucket pipelines:

Uploading tarball of [.] to [gs://PROJECT_cloudbuild/source/1645481495.615291-f6c287df7e6d4fd8991bed1fe6a5b9ca.tgz]
ERROR: (gcloud.builds.submit) PERMISSION_DENIED: The caller does not have permission

I don't know which permission I am missing - if anyone can give any pointers on this, that would be awesome!