When you add a new "Pearson MyLab & Mastering Link" activity in Moodle, it will give an error message when clicked on. This is because the link activity's settings need to be edited slightly from their defaults, due to a version incompatibility in Moodle's LTI interface. This issue can be fixed with a simple text edit that only takes a moment to do for each activity that's not working.
See also: