Hi @Peet,
Welcome to the RStudio Community Forum.
Ideally, you should provide us with more information so that we can help you (please see the posting guide).
I'm guessing that you are using RStudio. Go to the "Tools> Global Options> Packages" menu and click to "Change" the Primary CRAN Repository. Select one from the list that is close to your geographic location. Then try to install a package, such as:
install.packages("tidyverse")
If this fails, check that your machine has access to the internet and is not restricted by antivirus, firewall, or use of a proxy server. You must also have user permission to install new software on your machine. If not, see your System Administrator.
HTH