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.