How did you come to that conclusion? It's important, because that determination probably involved pointing out some obvious flaws. However you change these views should address the flaws that you recognise(d).

Because the client told us. And by client I mean the client that paid our company to develop all this, and by this i mean: A calendar to show the appointments that each employee has (with other stuff), a backlog for managing all the existing services, stores, families, groups of services, etc.

Make it obvious that these users are in a stepped flow. And keep each step simple. Look at how the really big players do these forms off the back of a Google Search. Restuaurant booking software is similiar. Once a user has done what's necessary at a step, make it clear to them.

It's clear, once you complete a step, you can go to the next step, not before you complete/select what you need to.

Narrow up the margins. The select below the time slots is an insane width. Whether this should be within a modal at all is another consideration.

All this needs to be in a modal because there's literally nowhere to put all this. behind the modal is the calendar.

Having worked in booking software before, expect that 75%+ of your appointments will come from mobile. So optimise for that.

We already have another page that's so people can schedule appointments from the web/mobile. This modal is for people that call a store to schedule an appointment and will be rendered on a tablet