What am I doing wrong in my script? (I want to make it so that when entering the trigger, the player's speed increases)
Take a look at Eskil's eat-drink script in community content. His system goes through a lot of steps (get the drink, drink the drink, etc.) to change a player setting, but I imagine that the trigger can be changed to not require player input and the script changed to go directly to the increase in speed.
I'm fairly new to the editor but from what i can see here you should get an error because you put an uppercase M in player.maxWalkSpeed. I don't know more, haven't tried triggers yet.