How do I fix an intermittent line on my iPad screen?

You can easily determine if you have a hardware or software issue. A 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 line is not present in the screenshot image, then you have demonstrated that you have a hardware fault that, sooner or later, may become sufficiently severe that 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.

Note: When reviewing the image, rotating the iPad screen orientation by 90°, from Portrait to Landscape, will ensure that the image does not coincide with screen defects if present.

Take a screenshot on your iPad – Apple Support

If the test indicates that a hardware fault a visit to the Genius Bar at your local Apple Store is recommended - where the technicians will be able to hands-on test and assess your iPad. This will likely inform your next steps:

Genius Bar Reservation and Apple Support Options - Apple

iPad Repair – Official Apple Support

That's not much information to go on. If you want help please provide more information.

Perhaps reading this article before replying will help: Writing an effective Apple Support Commun… - Apple Community