This is one of the extremely rare occasions where you need a semi-colon (other than a var statement).
Because of the parenthesis around (target), GML is interpretting:
sprite_index=spr_petiteexplosion
(target).HP-=damage
as
sprite_index=spr_petiteexplosion(target).HP-=damage
where spr_petiteexplosion(target) is being incorrectly attributed as a function.
Your solution is to insert a semi-colon after your sprite_index statement.
sprite_index=spr_petiteexplosion ;
(target).HP-=damage
And like Tsuka said, don't print screen shots and post actual error messages. The error message should not have suggested that the sprite spr_petiteexplosion doesn't exist but rather a function or script spr_petiteexplosion doesn't exist.
I would also spend some time learning about proper use of semi-colons as statement separators.
Edited by NakedPaulToast, 10 July 2015 - 07:49 PM.