I don't have Quick Sampler and don't know how to add it. How can I fix this?

This seems to be the relevant support document:

https://support.apple.com/en-us/HT210972

I don’t have Logic Pro, so I can’t say much else, but check that you have a recent enough version of the application, and follow the steps in the support document and let us know.

For more specific help about Logic, you probably should post your question in the Logic Pro forum. You are not likely to find many Logic users here in the Catalina forum.

There is nothing in macOS named Quick Sampler, and never was. Did you mean Quicklook? It still works just as before.

Or maybe this a third party application that you have to download. Please clarify.