It all rather depends on where you are trying to select text to copy. For example, in the Safari web browsing app, you will come across some web pages that will not let you copy anything from them. I don't know why - it's a coding thing built into the web site by the site owner. But, it's not you or the iPad in this case, it's the web site.
So, what app are you using that is not letting you copy text? Then, we can see if it occurs on our devices (if we have the same app).
Marilyn