I am trying to select the first device from popup to cast using selenium, capybara but I am not able to do it. Please find the popup below
I have tried following and it does not seem to be working
popup = page.driver.browser.window_handles.last
page.driver.browser.switch_to.window(popup)
page.driver.browser.switch_to.alert.accept
page.driver.browser.switch_to.alert.dismiss
page.driver.browser.switch_to.alert.text
Please provide sugggestions!

You can't - that's a system dialog and selenium can't interact with it once opened. It MAY be possible to set a default to launch and prevent the dialog from opening via chrome command line switches but I'm afraid I don't know what that would be.