I guess from the scenario and the approximate timestamp that this happened between 2016-07-21 18:07:57 and 2016-07-21 22:01:15 UTC. Please mention this information explicitly, it's important!
There was a pending suggested edit at this time. When a suggested edit is pending, no other edit can be made.
When you use the “full” website:
- If you have less than 2000 reputation, the “improve” link is grayed out and a tooltip explains why.
- If you have at least 2000 reputation, clicking on the “edit” link popus up a screen where you can review the suggested edit, and you have the option to reject or approve it immediately and make your own edit in the same go (“Improve”, “Reject and improve”).
It seems that the mobile website doesn't display the “edit” link at all. The Android app (I don't know about the iPhone app) has the same problem. Stack Exchange's policy is that it's acceptable to have some features missing on the mobile site and in mobile apps, and reviewing is one of these features. But it should still explain why editing is not possible (e.g. keep the “edit” link and pop up a message with an explanation and a link to the question on the website).
This bug has been marked status-bydesign on the main meta.