MD 513LL/A WILL NOT UPDATE THE 15.6.1 UPdate. Is this a problem ?

That model number corresponds to a 2012 iPad 4th generation.

This is exactky your issue.

That iPad will be 10-years old very soon!

It is too old and has not been supported by Apple and third party app developers since 2016/2017.

That year and model iPad is obsolete.

Sorry.

While this old an iPad maybe vulnerable, it may not be at any immediate risk from any hacks, security threats or any other similar security vulnerabilities that would affect your device and its data.

This security vulnerability would need to, directly, target your individual device as a point of attack.

With such an old device and unless you know that you and your device could be, possibly and directly, targeted, a device security attack will be highly unlikely.

In addition, the internal, rechargeable batteries in these old iPads are, also, running on borrowed time.

Not worth the costs to replace these old internal, rechargeable batteries in these old iPads, any longer, either.

Also, these very old iPad models have NO real trade-in nor any resale value, either.

Sorry.

It is almost the Fall season of 2022, now.

iOS/iPadOS 16 will be releasing soon, from Apple.

It maybe time to seriously consider a purchase of a new iPad model OR a much, MUCH “newer” and less expensive, refurbished or used iPad models, from better known, trusted sources, like other Apple product/device retailers/resellers, some locally or from online, on the Web/Internet, capable of running the latest iPadOS versions.

There are PLENTY of newer, older iPad models that can run the latest, current versions of iPadOS.

If you need OR ONLY able to purchase older, less expensive iPad models, look at older IPads from 2015 and later.

All of these older model iPads below CAN, currently, run the latest releases of iPadOS 15 and the up and coming iPadOS 16, releasing sometime in the Fall of 2022.

2015-2017, 12.9 inch screen iPad Pro models. ( 1st gen Pencil support )

2017, 10.5 inch screen iPad Pro models ( 1st gen Pencil support )

2017, 9.7 inch screen iPad 5th gen models ( NO Pencil support )

2018, 9.7 inch screen iPad 6th gen models ( 1st gen Pencil support )

2019, 10.2 inch screen iPad 7th gen models ( 1st gen Pencil support )

2020, 10.2 inch screen iPad 8th gen models. ( 1st gen Pencil support )

2018, 11 inch or 12.9 inch screen iPad Pro models ( 2nd gen Pencil support )

2019, 10.5 inch screen iPad Air 3rd gen models ( 1st gen Pencil support )

2019, 7.9 inch screen iPad Mini 5th gen models ( 1st gen Pencil support )

Get the largest internal data storage model you can afford as NO Apple iOS device has ANY expandable storage.

I am sorry that you had to learn all of this, but this does not change any of the facts regarding these older iOS devices.

Sorry & Best of Luck to You!

If you decide to go down the path of purchasing used, secondhand Apple devices, here is how to avoid purchasing an Apple iOS/iPadOS device with user activation lock, in the future.

If you purchase any iPad, or any Apple iOS device, for that matter, used/second hand, online, or even in-person, make sure this iPad does NOT have user activation lock and has been properly reset for sale.

A properly reset for sale iPad should initially power up to an animated, multi-language “Welcome” screen.

DO NOT take the iPad seller's word that this iPad is user activation lock free!

We have seen plenty of used iOS device purchasers get scammed.

User Activation Lock can never be bypasss, circumvented or worked around IF there is no correct previous/original user information to be able to do so.

Apple doesn't get involved with ANY user activation locked, used/second-hand sold devices, either.

So, buyer beware!

If purchasing a used device in-person, power the iPad ON

If the iPad DOES NOT display the initial animated, multi-language “Welcome” screen, then pass on purchasing this iPad.

If the iPad powers up to any other screen, then forget purchasing this iPad.

You cannot trust that the seller actually has the correct needed info to properly reset the iPad back to factory settings right then and there before the actual purchase and exchange of money for the iPad, in question.

Pass on the device!

If you are purchasing this used iPad online ask the seller to provide multiple images, either online or be sent to you via email, showing this animated, multi-language “Welcome” screen of the interested in purchasing iPad model.

Even better, ask for a short video be sent to you showing this animated, multi-language “Welcome” screen, in action and in its entirety, posted to the online ad or emailed to you.

If the online iPad seller refuses any of these requests, pass on the iPad. DO NOT buy!

Good Luck to You!

That is because an MD513LL/A is a 4th Generation iPad that is obsolete. Due to hardware limitations, it can run no higher iPadOS than iOS 10.3.X