@liquid-shane-o the old fbalpha core hasn't been part of RetroPie for more than 2 years, as noted in this topic. If you still get this error, you're probably using a very old and out-of-date RetroPie version.
Never mind. Checked on Reddit and got a hit for the problem I believe. Just used the PowerBlock installer in the setup menu. Seemed to work at a glance.
Does anyone have any recommendations on the best two player team games? - The wife is loving Tumblepop - has completed it three times now... but I got bored quickly...