Currently, Deployment Pipeline has a Limitation where you cannot add/change the Pipeline assigned to the workspace if the user has not been given the rights on the pipeline.
You should have Administrator Rights to run the Rest API for that workspace and azure directory ( Basically a Power BI Admin)
Problem :
Someone assigned the Workspace to his/her pipeline and left the organization.
Did not add any other user/group to access the pipeline as per the recommendations.
There is no way you can Assign yourself or your group to Pipeline and fix it.
Since starting to use Power BI I have been advocating the following 'basic' model
However. You can also create reports within Power BI Service. This is mentioned within the certification route for DA 100 Analysing Data with Power BI. Which makes things a little more confusing and I have been asked on lots of occasions. "If you can do everything in Service, what is the point of using Desktop?"