Dynamic Membership Rules -> Add Expression disabled

[MosesLim]

I would like to update my current dynamic AAD group by configuring some additional rules.

But I noticed that the + Add expression is grey out. 

I'm the Global Administrator, and my AAD License is Azure AD Premium P2. Its configured with AD Connect to an On-Prem AD.

Did i need additional permission or are there any conditions?

[#2]

That seems to depend on the type of rule you have, for example if you use something like the built-in "All users" rule, the button is grayed out. But you should still be able to Edit the rule via the corresponding button.