Unable to sacrifice k'rrik to cast a spell and use his ability to pay for black mana for that spell
When casting a spell with sacrificing a creature as an additional cost, you are should be able to select k'rrik to be sacrificed if you used life to pay for black mana in that spell's cost. As you pay costs in any order, this means you can pay the mana(and life) part of a cost before the sacrifice part(or the other way around if you really want, which arena also doesn't allow you to do. This can matter if you want to sacrifice vilis to a spell while also paying life for said spell with k'rrik if you don't want to draw cards and potentially die.)
However, arena does not let you do either of these and greys out K'rrik as a selection for said spell if you paid life into it and also forces you to pay life for the spell before sacrificing instead of letting you choose to sacrifice before paying the life. Both of these are incorrect and lead to some very unfortunate situations in K'rrik decks which have some interactions designed specifically with these two things working correctly in mind.
Say you have a card with casualty and want to copy it. If k'rrik is your only creature and you pay life with k'rrik's ability, arena will not let you cast the spell and sacrifice K'rrik.
Or say you have a card that sacrifices a creature as an additional cost and draws cards and you used a card that gives k'rrik the ability to return itself to the battlefield when it dies. If you use life to cast the draw spell, you will be unable to sacrifice K'rrik to cast the spell, negating the spell to reanimate it when it dies.
Here is the relevant rule in the CR that supports what the proper outcomes should be:
"601.2h The player pays the total cost. First, they pay all costs that don’t involve random elements or moving objects from the library to a public zone, in any order. Then they pay all remaining costs in any order. Partial payments are not allowed. Unpayable costs can’t be paid."