I have already installed Shiny, I have changed the R version, the mirror, everything, but the mistake is always there!
Loading required package: shiny
Installing package into 'C:/Users/user/Documents/R/win-library/3.6'
(as 'lib' is unspecified)
Error in contrib.url(repos, "source") :
trying to use CRAN without setting a mirror
Calls: ... withVisible -> eval -> eval -> install.packages -> contrib.url