what does this mean?

oh, there appear to be no attachments on this fancy forum.

well it says:

Assertion failed: Could not find file extension when trying to load an animation from external bitmaps!

what's it mean and how do i fix it?

I got it while trying to code my ice moon mod

I think that you had an image that wasn't in the cortex command directory. Just place the image back in the directory that you're working on.

I had that before it has something to do with an actor and frames one of your actors (or guns) is set to use frames but it does not have enough .bmp image files for all the frames there are 2 ways to fix this.
1: Find the actor with too many frames and set his frames to 0
2: Find the actor with too many frames and create the required amount of frames eg Actor000.bmp Actor001.bmp Actor002.bmp

I think that is how to fix it sorry but I dont know the exact name of the variable that says the framecount ... I think that's it Framecount.