I'm making a Cleric Domain with Potent Spellcasting. Everything looked great when I tested it, but I just noticed there is no option to replace PS with Blessed Strikes in the character creator for those who might want it.

Does anyone know what I might be doing wrong, and how I can fix it?

You’ve done nothing wrong, most subclasses don’t offer that choice, it’s simply one of the other as dictated by the subclass. If you want there to be an option for the player then there’s two ways you could do it, either as a “Class Feature Option,” or as an “Optional Class Feature,” which are not the same thing (confusing, I know):

1. Class Feature Option. If you specifically want the class feature to give the player that choice, then you would have to change your class feature to present that choice and implement the two as Options attached to the class feature. I explained how to properly set up Options in this thread; (Subclasses & Races FAQ #1): https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq
2. Optional Class Feature. If you want the choice to exist but not be explicit, then you would create Divine Strike as a “Replacement” class feature set to replace Potent Spellcasting and it will work just like the optional class features in Tasha’s Cauldron.

I hope that helps.