hey mate, i guess u are using the first person ue Template right? then u have all those animations (weapon holding etc) for the skeleton of the template mesh (this robot guy thing)
it would not work if you just replace this mesh , because of its skeleton …
e.g: you want to fit the skeleton of a elefant into a fox’s body ^^
u can try “retargeting” ur mesh (`there are plenty of tuts out there) when it has the same bones, or use ur own animations … i guess there is no simple drag and drop solution for that ^^