Welcome to the community, @Rita_Roloff! :grinning_face_with_big_eyes: If you copied that formula directly from your formula field, the most glaring issue is the space between IF and its opening parenthesis. It should be:
IF(...
not
IF (...
The same goes for all of Airtable’s functions.
If (no pun intended) that’s not the issue, the next thing to check are the quotes. If you had copied a formula from elsewhere in the community, for example, it’s possible that you captured some “curly” quotes. Airtable is picky about quotes, and only works with non-styled “straight” quotes in formulas, as this cleaned up version of your formula uses:
IF({Num Porosity} >50, "High Porosity", IF({Num Porosity} < 20, "Low Porosity", "Normal Porosity"))
