What is the Version Number of your Lightroom?
What device are you using? (Laptop, iPhone, iPad, Android)?
"Impossible"- Why? Do you get error messages? Does anything happen?
What "format" are you choosing?
Regards. My System: Lr-Classic 12.3 Photoshop 24.3, ACR 15.3, Lightroom 6.3, Lr-iOS 8.0.8, Bridge 13.0.3, Windows-11.