My Mac computer had a horizontal line. How to remove this

Hello!

This is accidental hardware damage, and it must be fixed by Apple. Get started here: Get Support.

Only go to Apple, an Apple Authorized Service Provider, or send your device to Apple for service (if applicable or available). Do not go to a 3rd party/unauthorized repair center or attempt a repair yourself. This is for the assurance of authorized and professional repairs that use genuine Apple parts. Please refer to this from Apple Support:

Beware of counterfeit parts

Some counterfeit and third party power adapters and batteries may not be designed properly and could result in safety issues. To ensure you receive a genuine Apple battery during a battery replacement, we recommend visiting an Apple Store or Apple Authorized Service Provider. If you need a replacement adapter to charge your Apple device, we recommend getting an Apple power adapter. Also non-genuine replacement displays may have compromised visual quality and may fail to work correctly. Apple-certified screen repairs are performed by trusted experts who use genuine Apple parts.

Jack

This could be a failing GPU, please try restarting the computer in Safe Mode and then restart normally and check.

What is the model year of the computer? If you are unsure, please click the symbol in the upper left corner of the screen and then click About this Mac and it will tell you the model year.

If a Safe Mode restart does not work, your next stop is to contact your local Apple Authorized Service Provider and take the computer in to be professionally diagnosed.

Definitely a bad Display Assembly which will need to be replaced. Unable to tell from the picture if it is caused by accidental damage.

FYI, those are vertical lines. Hard to tell if there are also horizontal lines beneath the Dock.