How can I get my Mac Mini to move my mouse between two screens? ?

I'm using Catalina and I've just added a second monitor with a HDMI cable. The screens work but I cannot one my mouse off the main screen to the second one. I've gone into System Preferences | Displays and made sure Mirror Display is Off and I've arranged my screens. But still I cannot use my mouse on second screen. How do I fix, please?

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