I need to upgrade my iPad to OS12

Look here and identify your model

Identify your iPad model - Apple Support

Then we can tell you if it can be updated.

It may not be. All devices eventually hit a point where they cannot be updated any further.

You might need to update, but the question is whether your iPad is able to update.

As you’ e not provided any details of your iPad model, or the currently installed version of iOS, nobody here will be able to give you definitive guidance as to why you are unable to update - or even if you can update at all.

If you can provide more information, we may be better able to help you.