Even though squibs cannot work magic at least as with wands, we know that they aren’t completely bereft of magical potential– they can for instance see magical entities that are invisible to muggles. So could a squib mix a magical potion if as Professor Snape said “There will be no foolish wand-waving or silly incantations in this class,”.
My understanding Squibs can do no magic. The are born into Wizarding families but possess no powers.
They are magic-aware and see things.
So, no they cannot mix potions, as written.
Curiosity got the better of me. Apparently you have to wave your wand over the potion as the last step. But this is only stated in supplemental material.