You posted your question in someone else’s thread in the US citizenship process forum. That post was split off into its own thread and moved to the correct K1 forum.
To answer your original question, here’s the information from Visa Journeys portal for Spain which other users update for any pertinent information:
“There was about 1 week from when the consulate received the case from NVC until "packet 3" was emailed. We also recieved a paper copy in the mail, which took two weeks. Once your paperwork is sent back, the embassy will review them and email you "permission" to move to the next step and schedule your interview - this took exactly one week from when they received our paperwork. You are then "allowed" to log into the website they provide in packet 3 to choose a courier, pay the visa fee, and schedule your appointment. When I initially logged in, the embassy showed appointments available between 2-4 weeks out”
https://www.visajourney.com/consulates/index.php?ctry=Spain&cty=Madrid
There is portal for every country on VJ. This update above is from 2017 so maybe there are delays in wait times but it seems you need permission from the Madrid consulate to schedule an interview. Did you receive such permission or no? If not, then you need to wait for that.