I do not recommend that you add the entire item on one line. Instead, do this:

Code (Skript):

1. command /give:
2.    trigger
3.        set {_axe} to diamond axe
4.        enchant {_axe} with unbreaking 10
5.        enchant {_axe} with efficiency 8
6.        enchant {_axe} with sharpness 5
7.        enchant {_axe} with mending
8.        give {_axe} to player

 

That's weird.

Try setting a name to the axe

Code (Skript):

1. set {_axe} to diamond axe named "&4Powered axe" #for example.

 

I tested it myself and it works fine. Btw you just messed up the skript, here is the correct one:

Code (Text):

1. command /test:
2.     trigger:
3.         set {_axe} to diamond axe
4.         enchant {_axe} with unbreaking 10
5.         enchant {_axe} with efficiency 8
6.         enchant {_axe} with sharpness 5
7.         enchant {_axe} with mending
8.         give {_axe} to player

You only messed up some ":" and some spaces... What version of skript are you using? And what are your addons?