when I tried the skript it came up with 5 errors, are you using an add-on?
(also all of the inventory space checking things say diamond)

 

You must continue the chain of "else". For example:

Code (Text):

1. command /do [<text>]:
2.   permission: blahblahblah.procrastination
3.   trigger:
4.     if arg-1 is "argumentoneedition":
5.       do this
6.     else:
7.       if arg-1 is "argumenttwoedition":
8.         do this too
9.       else:
10.         if arg-1 is "argumentthreeedition":
11.           do this as well

sorry I don't use tabs, I prefer soft, minimalist spacing
you can also use "else if" and then an event if you want to save spacing

 

Code (Text):

1. if arg 1 is "lapis":
2.                 if the player's inventory doesn't have space for diamond:
3.                         send "&7[&4&l!&7] &c&lYou don't have enough space in Your Inventory!"
4.                         stop
5.                 if player's balance is more than 45:
6.                         remove 45 from player's balance
7.                         send "&7[&2&l✓&7] &2Successfully Bought &9 16x Lapisu"
8.                         add 16 lapis lazuli to player's inventory
9.                         stop

you put if the player's inventory doesnt have space for diamond? is this a typo lol?

 

im pretty sure its just that he didnt read the docs beforehand