I made a 6 beacon pyramid out of iron blocks then I was putting iron ingots into the pyramid and 5 out of the 6 worked fine, Then on the 6th one I selected regen and when I put iron in the checkmark was still greyed out.

(Edit) I solved the problem but it's still an issue. You have to select another potion effect (Haste,Speed,Jump etc.) Then you can put the iron in and click regen and click the check.