Hi, and welcome!
Please see the FAQ: What's a reproducible example (`reprex`) and how do I do one? Using a reprex, complete with representative data will attract quicker and more answers. Not necessary for this question, however.
Very few packages are standalone, and tidypaleo is not exception. The description file shows a ton of packages that it relies on. These are dependencies.
So, this happened because the install for the package tried to install the dependencies, as well.
The immediate reason is that the R configuration file on your page pointing to http://cran.rstudio.com/bin/macosx/el-capitan/contrib/3.5/PACKAGES is trying to open a 404 (not found) page.
If you are installing within RStudio, see the configuration guide. If you are using the MAC R GUI, see the Packages & Data | Install and choose another repository.
However, you'd save future trouble by updating from R.3.5 to 3.6.
Finally, if you run across "source version is later" and respond "yes," install from source, if it doesn't install the first time, there's nothing to be done by most users, because it requires editing the underlying source code to run on Apple's non-standard compiler.