Why can't I share Magic Mouse between MacBook and iPad

Share how exactly?

With universal control, you can drag the mouse off the side of the Mac and onto the iPad and click and control things on there.

What exactly is not working?

click here ➜ Universal Control: Use a single keyboard and mouse between Mac and iPad - Apple Support

Connect your devices

After turning on Universal Control, you can connect (link) each device:

1. Put your devices near each other and make sure that they're awake and unlocked.
2. Use your mouse or trackpad to move the pointer past the left or
3. right edge of your screen in the general direction of the iPad or other
4. Mac, even if the other device is positioned below your screen. You can arrange your displays more precisely later.
5. As you move the pointer past the edge of the screen, the other
6. device shows the pointer beginning to push through to its screen. To
7. connect to that device, push the pointer all the way through.
8. 

Make sure the following is correct:

• Each device must be signed in with the same Apple ID using two-factor authentication.
• Each device must be within 10 meters (30 feet) of each other and have Bluetooth, Wi-Fi, and Handoff turned on. 
• Your iPad must not be sharing its cellular connection and your Mac must not be sharing its internet connection.

So it does not move over to the iPad when you move it to the very edge of the Mac screen?

Try the other edge of the Mac screen and see if it starts the indication as shown the image above, and then pushes through.

Without seeing your screen and settings its hard to say why its not working or what may be missing.

Double check your are using the same Apple ID and 2 Factor authentication is enabled on your Apple ID.