Do the following, which I found on here:
I ended up ringing Samsung and the problem is fixed. They changed the way to access the multi-window feature. First, you need to press the bottom 3-line button once and the list of opened apps will appear. Then press on the icon at the top of the app you wish to split screen and select from the menu 'Open in split screen view'. Its a tad more difficult and somewhat a hassle but at least they didnt remove the feature from the system."
It takes more effort, but it worked for me! Good Luck 😊
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