While 5.1.1 is technically "above" 5.0, WhatsApp frequently updates its encryption and features, often making the app unstable or impossible to install on hardware from that era (typically devices from 2014-2015).
This method will likely fail if WhatsApp has dropped support, but it is worth trying first. whatsapp free download for android 5.1.1
Are you running into a specific or storage issue while trying to install it? While 5