I bought an iPad online and it worked, now it is showing an activation lock. The person I bought it from 4 years ago I have no way of getting up with. How can I clear this up? Serial numberF5RKTH7ZDFHW

You have a problem - as without the assistance of the original owner, the iPad is now useless to you.

Here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be turned off:

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Turn off Activation Lock – Apple Support

As you will discover, unless you are the owner of the AppleID that was last used to activate the device, you’ll not be able to defeat or bypass the Activation Lock. With documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple may be prepared to assist - otherwise they and nobody here will be able to help you.

Here is what Apple suggests when purchasing used:

Check for Activation Lock before you buy a device from someone else

Before you buy an iPhone, iPad, or iPod touch, from someone other than Apple or an authorized Apple reseller, make sure that the device is erased and no longer linked to the previous owner’s account. Follow these steps to check if the device is ready to use: 

1. Turn on and unlock the device. 
2. If the passcode lock screen or the Home screen appears, the device hasn't been erased. Ask the seller to completely erase the device by going to Settings > General > Reset > Erase All Content and Settings. Don't take ownership of any used iPhone, iPad, or iPod touch until it's been erased.
3. Begin the device setup process.
4. If you're asked for the previous owner’s Apple ID and password, the device is still linked to their account. Hand the device back to the seller and ask them to enter their password. If the previous owner isn't present, contact them as soon as possible and ask them to use a browser to remove the device from their account. Don't take ownership of any used iPhone, iPad, or iPod touch until it has been removed from the previous owner’s account.

You will know that a device is ready for you to use when you're asked to "Set up your iPhone," “Set up your iPad," or “Set up your iPod” the first time you turn it on.

You don’t.

That iPad is, now, a dead useless device to you.

Without the aid of the previous owner of this iPad, that iPad will never, ever work for you, any longer.

If this is a very old iPad, it maybe obsolete, anyways.

If you never made ANY data backups of this iPad to Apple iCloud or a full computer, then the data that was on this iPad is gone forever.

Sorry.

If this is a very old iPad, please properly and responsibly recycle this device, now.