Please show a complete error message by copying and pasting from the terminal, including the Pip command(s) that you use and all the output you get, and format it like multi-line code following the advice in the pinned thread. This will allow us to confirm what actually went wrong.
The most likely reason is that the library you want to install does not support the version of Python you are using.
The error that you mention looks like one that comes from Pip. Are you, perhaps, using an IDE such as PyCharm, and asking it to set up packages for you? That will run Pip for you behind the scenes; and in many cases, a PyCharm project will try to run your code (and install third-party libraries) for a separate Python environment - it may not be the one where you manually installed the library.