In certain scenarios (for example, when you changed your K2 administrative accounts) you may see the following error when trying to add or remove Environment Field in Environment Library:
This may happen even for user which has been assigned K2 Administrator role in Setup Manager when custom security was configured on Environment Library and it didn’t include this specific account.
To resolve this (providing you have account with administrative rights) just look into Security settings available under list of variables themselves when you navigate to Environment Library > %Environment Library Name%:
Just add required user assigning him Modify rights to resolve this issue.