if (!requireNamespace("devtools", quietly = TRUE)){install.packages("devtools")}
devtools::install_github("jbisanz/qiime2R") # current version is 0.99.20
This comes down to some issues with dependencies and its hard to troubleshoot just from that last message as every case is a bit different.
Can you try restarting your R sessions and manually installing the devtools package?
install.packages("devtools", dependencies = T)
If that is successful then you can just move on to installing qiime2R
devtools::install_github("jbisanz/qiime2R")
If for some reason it is not successful, pay attention to the error message, it will likely name some packages that it couldn't install for some reason, at which point you'll want to manually try installing those one by one.