When I select a channel and press PTT button, it won't let me speak
Only one person at a time can speak on a channel. The first person who speaks gets the "floor" for up to 20 seconds or whatever time interval has been set by the channel owner. The time interval can be set from 0 seconds to 10 minutes. During that time, other users cannot interrupt.
If you can’t speak, release PTT button, wait for the other person to finish and try again.