Can you be explicit about how you are exporting to beamer and what version of org you are using? What happens if you start emacs with -Q?
For me, all I require is to have (require 'ox-beamer) and then I can export with "C-c C-e l O", having started emacs with -Q. -- Eric S Fraga via Emacs 27.0.50, Org release_9.1.14-1035-gfeb442
