Hi @sajadtasharofi,
Welcome to the RStudio Community Forum.
Try using a different CRAN mirror site, preferably an https: one. In RStudio, navigate through the Tools>Global Options>Packages menu items to change the "Primary CRAN repository". the change, and then try to install the dslabs package again (using either install.packages("dslabs") or via the RStudio menus.
HTH