Recently, I faced this issue (My Mac was almost filled): "Model could not download due to insufficient Memory," not the exact message (I didn't screenshot it).
I already did this process before and have the model downloaded
I don't know why the model was deleted
I don't know when it got deleted (I used it 2 days ago)
I deleted stuff from my Bin and tried running "await LanguageModel.availability()" in my Chrome console, and was still getting "unavailable"
Restarted my Mac and tried again, it started showing "downloading" and after a few seconds it finished downloading.
So, if you encounter such, try restarting your computer.