Hey there! Below are some really helpful links from the gamedev community that help with Baking in Blender pretty often from what I’ve seen.
WIKI - Interesting forum topics, problem solving issues - Blender Courses / Talk - GameDev.tv
There’s a section in this link above specifically subtitled “Problems when baking” try checking those out. The very first link in that section, NP5 gives a nice post with steps to solve a majority of baking-cavity issues. If you have trouble finding it still let me know.
This helped me whenever I got stuck on the same section, so give it a shot, but if you still can’t get it working, please let us know and maybe someone can help with the file directly. It took me a little over an hour of tweaking the details even after reviewing the link and pages, so take your time.