While, I MAY be wrong. These are a few things that might be going wrong.

First, you didn't define the entity. So you should put ENTITY* mangyaenemy at the top if you didn't already.

Also, if you are going to be using mangyaenemy for that function, you should just make it from:

mangyanenemy = ent_create("mangyan.mdl", vector(0,-100,-10),NULL);

to

ent_create("mangyan.mdl", vector(0,-100,-10),func_backselectenemy);

and then turn the mangyaenemy to my.

Also, pan_backselectenemy=pan_create(" doesnt see, to be complete...

Also, remember. Sometimes the engine will point at random lines of code even if there is something else wrong with a different time, like if you forgot to put ; at the end of a line.