The error message is a bit of a misnomer. It is actually telling you that it can not find a package called QIIME2R in CRAN. Broadly speaking, there are 3 repositories you will use to install packages for R: CRAN, Bioconductor, and Github. CRAN is the general repository for packages which is where
install.packages() installs packages from by default. Bioconductor is where many bioinformatics tools are located and is where
biocLite() pulls its packages from. Github is usually where the latest development versions of packages or new packages that have not yet gone into a major repository go (like qiime2R). To install from github, the easiest way is using
devtools::install_github(). While I plan on submitting qiime2R to bioconductor, for now it is only available from github. You can install it as below:
install.packages("devtools") #if you don't have devtools installed
With all that said, it does specify that your R version should be 3.5 or greater in the current version which is a bit artificial but what can I say, update your deal.