You are using the wrong object.
You have to use the " wooden fence", not the "wooden wall" from the mining theme.
To be more general, you have to use a fence and not a wall because in RCT2, only the fences can be placed sloped, the walls can't (there are some exceptions).
Hi, the objective in this scenario is having 600 guest, i don't know why your goal have changed.
On the other hand, reach 1200 guest in two month is not difficult. Just make the park free to enter and charge 5 or more the roller coasters, and then just spam ad campaigns.
Hope this help you, and sorry if something is misunderstood, English is not my native language.