If you open the barracks via the construction or production list, you can not enable or disable the barracks.
This can currently only be done via the buildings listing on the province screen, and then clicking on the barracks icon.
Then the info screen will pop up and if you then press enable/disable it will work.

If you go there via the + construction/production (you have a tab buildings there too) you can go to barracks and get the info pop up too. But clicking on the enable/disable button wont work.

Regards,