Heat Instant pot to saute mode.
When hot, add butter and let it melt. Add chopped garlic into the pot and saute for 20 to 30 seconds, until it begins to get golden.
Press cancel and add chicken stock, salt and pepper. Give a good stir and please note that it is very important not to stir after this step.
Pour in heavy cream (DO NOT STIR) and add fettuccini on top, gently pressing the pasta to be submerged into the liquid (DO NOT STIR).
Place chicken on top in an even layer (DO NOT STIR).
Close and lock lid (making sure the valve is in sealing position). Select manual cooking mode and set for 6 minutes (for al dente pasta ) or 7 minutes (for softer pasta).
Once cooking time is done, let it be in warm mode for 7 minutes (or after 9 minutes for softer cooked pasta).
Release pressure and open lid and stir in parmesan cheese. The pasta will be liquidy at this point but will thicken as it cools (do not cover).If you are using a smaller instant pot and liquid sprays out when trying to quick release, then simply turn valve back to seal and wait for few more minutes before trying again.
Serve after few minutes with additional grated parmesan if desired.