I tapped download and install manually option in Windows Phone store and I downloaded an xap file. I transferred the xap file to my Lumia 435 via bluetooth, but the OS does not install it, displaying "Cannot install company app..." I checked out This Microsoft Thread on how to install xap files. It says, "Tap more option on apps list and then tap install local apps (or SD card)." I don't see any such option in my apps list.
The XAP file is signed using a certificate - most likely from Symantec. That certificate has a corresponding AET - Application Enrollment Token. You need to distribute the AET together with the XAP file in order to allow installation (Microsoft calls it a sideloading process).
Assuming this is XAP that you got internally, just ask the developer for the AET. if it is XAP that you created as a developer, find out how to generate it here