As stated at https://github.com/sdelements/lets-chat/issues/781 , the project is no longer developed since August 2017. Last version, 0.4.0, released in June 2015, can be still downloaded from GitHub together with the source code.
Our users have written 2 comments and reviews about Let's Chat, and it has gotten 37 likes
co-founder of Portal here - https://portal.cloud
We've been using Let's Chat for all our company communications for about 3 weeks now, and it's been awesome. It basically has all the functionality of Slack, without all the overhead, and I can self host it on Portal.
Try Lets Chat on Portal, just install it from the App Library.
Unfortunately development of Let's Chat halted: https://github.com/sdelements/lets-chat/issues/781