I set my Audacity up as was in the manual. I started recording and all seem well. Before I had recorded one half of my cassette the pause button came on and stopped the recording. Was unable to turn off either button.
You may have enabled Transport > Sound Activated Recording by mistake. Try looking there (in the menus at the top). If you see a tick (check) mark by that item that means it is turned on, in which case click on it to uncheck it and so turn the feature off.