basic use

trying to convert audio cassettes, when I press Record I get error message -9999 unanticipated host error, error opening recording device

Which version of Windows?
Which version of Audacity? (look in “Help > About Audacity”)
What cassette player and how is it connected to the computer?

windows 10
audacity 2.3.0
tape cassette I bought from you, Super USB Cassette Capture

We don’t sell cassette players. We don’t sell anything. We just make free software (our website:
Some companies include our software with their products, but we don’t benefit from that in any way.

There is some information in the manual about connecting up USB cassette players. See if this helps: Recording with USB turntables or USB cassette decks - Audacity Manual

If you still have no joy, let us know how far you got and we’ll see if we can help.