We would get the below error while trying to modify states in the Azure DevOps process.
VS402356: You do not have the permissions required to perform the attempted operation on this process
To fix this we can add the user to the Project Collection Administrators group.
Or better give Process specific permission to the user
As a result the user can now add and update the process.
Hope it helps..
If you see the classic editor option missing, while creating the pipeline, it is because it would be disabled in the organization settings.
To enable it navigate to Organization Settings >> Pipelines >> Settings 
After switching off the settings, we can see the classic editor option.
Get more details –
Hope it helps..
Recently one of our users was getting the below error while trying to access the VSTS Project.
401 NOT AUTHORIZED and the following account does not have sufficient permissions to complete the operation. The following permissions are needed to perform this operation. View project level information.
Here we can see the users being added to the project team and the team having all the appropriate rights.
Adding the user to other groups i.e. Build Administrators, Contributors, Project Administrators etc. also didn’t help.
Also, inside Manager Users section.
https://[[subscription]].visualstudio.com/_admin/_users
We selected the user and clicked on Manage Projects to check his access.
It showed the user being part of Project Administrators – VSTS Groups.
Still user was getting the same 401 Unauthorized error.
Not sure what happens in the background and it takes time for the change to propagate. We have faced this issue many times with certain set of our users (one thing we have noticed that these users had their Gmail or Yahoo id associated to VSTS). For some users it has always worked properly (one using Outlook, Hotmail and Corporate account)
Hope it helps..
Nishant Rana's Weblog 