Hi - I was getting exactly the same issue - but I was also being lazy - and had not setup the reverse-proxy and valid SSL.  Ie. I was only accessing via port 8069 on http not https.
I think actually the issue is the browser won't run the java script unless a secure site - well that's my guess.  Because I setup properly - created SSL.  Re-installed Website and Attendance apps and set the domain correctly for website and company. 

Set proxy_mode = True in the conf file.

Restart odoo.

Access via https and then everything worked fine.  That install button is actually to install the page as a webapp on a client kisok device.

Hope that helps - I was going crazy trying to work that out. 

you could also possibly try to whitelist and allow permissions for cameras specifically if not using HTTPS - but probably just setup SSL and use https.