PF2 does not have arcane spell failure at all. It does have an Armor Check penalty if you don't meet the Str requirement for the worn armor, this applies to Strength or Dexterity-based checks (that aren't attack rolls), so that means Acrobatics, Athletics, Stealth, etc. You also take a penalty to your Speed if you don't have the Str requirement for the armor.
Notably, if you wear armor you are not proficient with, you won't get to add your proficiency bonus to your AC. A witch or wizard (Trained (+2) in Unarmored, untrained in anything else) with a Dex of +1 gets an AC of 14 at 1st level (10 + 1 (Dex)) + 3 (Trained, 1st level)), 15 at 2nd, 16 at 3rd, etc. If you slap them in a Breastplate (+4 item bonus, max +1 Dex) that they don't have training for, they start at AC 15; that's 10 + 4 item + 1 Dex, but no proficiency bonus. Their AC doesn't go up with level as they're untrained in medium armor.
Classes start with armor proficiencies appropriate to the fantasy. You can take a general feat to add armor proficiencies (stacking, so taking it 2x on an Unarmored character gets you to Medium proficiency), or more likely use a dedication (think multiclassing) feat like Sentinel to add proficiency. Notably, the Rogue class dedication is fairly popular for this, as it gives Light armor proficiency as well as two trained skills.
So that example witch/wizard who spent a class feat on Sentinel dedication to get armor proficiency, now that they're at third level, now has an AC of 20 (10 + 4 item + 1 Dex + 5 proficiency (+2 for Trained, +3 for level)). If they didn't spend that feat, they'd still be at AC 15 in that same breastplate, but AC 16 without any armor at all!
Though they can cast spells all day every day in that armor regardless.
