Note: This is not "argh, Clerics are terrible"; judging particular classes is for other people. "Failure Cleric" refers to what is possible from the following.
1) There is no rule in PF2 stating a minimum attribute for spellcasting. In contrast, PF1 required your casting attribute be at least 10+slvl to cast a spell.
2) Unheightened Heal gives 1d8 + casting modifier healing, or just the modifier in the "mass" casting.
3) Heal does not have a rule specifying a minimum amount of healing.
Modifiers can be negative. You could potentially have a Cleric (or other divine caster) with Wisdom less than 10 who hurts people when they cast Heal. Also, because Harm is an exact mirror of Heal, the same Cleric would heal people with Harm.
If there is in fact a rule somewhere that contradicts the above, please point it out; if not, could they be added? (And no, the "starting at 10 and only having one possible -2 that is negated by the class ability boost" doesn't count. There's a sidebar about optional flaws and an entire page about roll "4d6 drop lowest" generation (which explicitly removes the class ability boost).)