Whenever I try to create a new model on singleplayer Sandbox, an error would pop up saying, for example,

[PAC3] ERROR: models/player/real/prawnmodels/pepsiman.mdl is not a valid model on the server.

Then whenever I move or crouch, it would just revert back to my original playermodel. I'm confused as to why it's saying the playermodels aren't valid considering it's singleplayer, is anyone else having this problem?