Currently there is no package that will run on the Pi. Now out of date - see below edits
The packages you are trying to install are for x86 chipsets only.
Best thing to do is move your day to day computing to the Pi and free up your PC or Mac to do the folding tasks.
Edit: Source for the client is not available for cross compilation
Edit 2: Balena (of Etcher fame) has now released a 'Fold for Covid' project image for the Pi 3 / Pi 4 and lots of other single boards / dedicated computers and 'spare' processing on your main machine. Full details are at THIS LINK HERE Note technically this is not Folding@home (it is based on Rosetta by Baker Labs but is for a good cause