Since the conflicted package exists at the location you specify, and since your syntax is correct, this is possibly related to a networking issue.
So my first question is whether this server can usually reach out to the internet? Are you able to install packages from CRAN? Can you ping the github site from this server (on the linux command line)?
The second networking issue you should check is whether you're using a proxy or not. If your proxy is cauing the problem, see the advice at 'install_github' gives timeout reached error · Issue #877 · r-lib/devtools · GitHub, specifically:
library(httr)
with_config(use_proxy(...), install_github(...))
OR
library(httr)
set_config(use_proxy(url = "******_", port = "_*"))
devtools::install_github("username/packagename")