What does StarterGui:SetCore must be called from a local script and how do I fix it?

I am trying to make an animation and I followed a tutorial because I am brand new, the output keeps sending this "StarterGui:SetCore must be called from a local script" and the animation is not working.

You have to put the code in a localscript. Cut and paste your code into a localscript. The localscript must be in StarterGui.

You need to use a local script for this, you are most likely using a server script. How to: Open Insert Object then insert a Localscript Into StarterPlayerScripts and write the code there.