hi
I am having the same issue for almost a year. I cannot remove it either from my iPhone settings or Mac setting. Mac gives me the same error that you have. In iPhone setting, it just rotate and go back to prior screen without it gets removed. I started apple support ticket in the past and no follow ups. I started another ticket last week and still waiting to hear from apple support. Despite several iOS & Mac software updates, changing the devices, resetting & restoring devices, the bug do not go away. Apple support in the past confirmed me that it has something to do with apple iCloud account level issue & apple has to fix it. it is not related to our devices, but somethings at iCloud account level issue. Hopefully apple support team will fix the issue in upcoming updates, but no idea when it will be fixed. It is just frustrating that apple software support team are not fast to fix these software bugs.
Thanks