It’s our old “friend” Rubygems giving a 429 error.
Lots of people have been getting that, no one is quite sure why. Critically, it has not been happening for Discourse’s own builds, so it’s not something inherent in Discourse. The people at Rubygems have not been forthcoming with an answer. The recommended “fix” is just try again in a little while. And maybe complain to Rubygems.
Search for “rubygems rate limit” or “rubygems 429” for other topics on this.