I cannot install 3rd party apk on my new sony led x8500e

[Android Central Question]

I cannot install 3rd party apk on my new sony led x8500e. Although I unchecked the properties in security section for 3rd party but when I try to install the 3rd party apk the install button on the window always disabled or some overylay it which I cannot see. Can anyone tell please how to install 3rd part apk? My andriod version is 7.0.