There have been several changes in the Atlas UI structure between Beta 2 and Beta 3.
Have you updated the Atlas UI module by the latest version from App store?
See the improvements section of the release notes about this:
I’ve just started following the learning paths on the Mendix 8 beta and got the same issue on both my work and home pc. I received a bunch of error messages indicating that Bootstrap couldn’t be found. Installing the Atlas UI from the desktop modular solved it for me.