So the way I am reading the rules it seems impossible to take a guarded step in Starfinder in difficult terrain.
So the guarded step rules say:-
You can carefully step 5 feet as a move action. This movement doesn’t provoke attacks of opportunity (see page 248), even if you’re in a threatened square (see page 255).
And the difficult terrain rules say:-
Difficult terrain, such as heavy undergrowth, piles of junk, or steep stairs, hampers movement. Each move into a square of difficult terrain counts as 2 squares of movement. Each diagonal move into a difficult terrain square counts as 3 squares. You can’t run or charge across difficult terrain. If you occupy multiple squares with different kinds of terrain, you can move only as fast as the most difficult terrain will allow. Flying and incorporeal creatures are not hampered by most difficult terrain, though a dense tree canopy or web of chains might count as difficult terrain for flying creatures.