How can I unlock the screen rotate on my i-pad mini 4 ?

See this article: If the screen doesn't rotate on your iPhone, iPad, or iPod touch - Apple Support

If it doesn't have a step that solves your issue, then you'll need to talk to Apple Support directly: Get started

Swipe up from the bottom of your screen (start a tiny bit outside the screen) and a menu should show up. From there look for the screen rotate icon (it's like a circular arrow) and press it once to disable it.