Greetings! If you are having trouble with the a button not working, I may have a solution for you to save you some time!
Currently on Unity 2021.1.12f1 using the new input system and was having some trouble while following along in the mobile game development course, currently working through the asteroid avoider section.
Make sure you update your Event system as shown in the video about Game Over in this section.
This link was helpful for many people: https://forum.unity.com/threads/cant-click-ui-buttons.431042/
For me, I had to go into the Event System and change the pointer behavior enum from its default (single mouse or pen but multi touch and track) to either “single unified pointer” or “all pointers as is.”
Look at the Unity docs : Enum UIPointerBehavior | Input System | 1.0.2
It seems the preferred method for this course is “single unified pointer”, but if someone with more experience would like to add more, they can.
Hope this helps someone!