Hi @peter_c ,
Do you want to assign a Security Role of your current Environment to a specific AAD Security Group Team?
Based on the issue that you mentioned, I have made a test, and the issue is confirmed on my side. It is an known issue with AAD Security Group Team -- when you want to assign a Security Role to a AAD Security Group Team, the "MANAGE ROLES" button would not be visible within the Top Menu Option of Teams List.
As an fixed solution, you could consider click the AAD Security Group Team, and open it, within the opening Team details Pop-Up, you could see the "MANAGE ROLES" button there:

Please take a try with above solution, then check if the issue is solved.
Best regards,