All files needed are from phosphosite resource, e.g.:

https://www.phosphosite.org/downloads/Acetylation_site_dataset.gz ,

They give a 403 error code on download (forbidden). The only way to download it now seems to be by going to the website and logging in.

I contacted the phosphosite resource a few weeks ago when I first saw this issue, but they never got back. So I guess their plan is to keep it as a private resource that is not usable for automatic downloads (at least without a login).