Please confirm that you select the proper context in Debug view. Please attach a screenshot of the entire IDE when button is disabled.
Also, please attach the .log file from your workspace. Maybe we can get a clue of the failure.
Please also provide answers to the following questions:
1. What IDE version are you using?
2. What board/MCU are you using?
3. Is the problem encountered with a new project created with the IDE?
4. When using the Develop perspective, is the Restart button enabled?
Greetings,
MCUXpresso IDE Support