Vertical line on my iPad after using split screen

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You can easily determine if you have a hardware or software issue. The reliable method to confirm a screen or other hardware fault is to take a screenshot when the fault is evident - and then review the captured screenshot image...

When reviewed, if the lines are not present in the screenshot image, then you have demonstrated that you have a hardware fault - for which repair or replacement of the iPad are the only options. If the lines are visible in the screenshot, the something other than hardware will be the cause.

If the test indicates that a hardware fault (as, shown in your attached photo) a visit to the Genius Bar at your local Apple Store is recommended - where the technicians will be able to test and assess your iPad and you can explore your available options:

Genius Bar Reservation and Apple Support Options - Apple

iPad Repair – Official Apple Support

Can you post a screenshot of precisely what you see - redacting the image if necessary (using markup tools) to obscure or remove any personal/sensitive information before uploading the image.

Take a screenshot - Apple Support

You can upload the screenshot image using the image button at the bottom of the message editing window: