Well, parrots always make flock calls early in the morning and again in the evening so she will always scream at those times regardless so do keep your windows close
But, if she used to living in an outdoor aviary setting with other birds, she is calling for them. Psittaculas are not very people-oriented to begin with and aviary ones suffer the loneliness much more than the ones that were handfed and don't know anything else -and even most of those are not 100% happy without mates when they are sexually mature. There are exceptions but that's what they are: exceptions. I would advise you to bird proof the room very thoroughly, put perches all around the place and let her out to fly - at least, that will distract her a bit because been in a cage, all by herself, indoors, and with strange humans must be real hard on her right now. Aside from that, you seem to be doing what needs to be done, namely, spending time with her in the same room, talking softly, etc.
Now, I am not saying this because I want to be the party pooper but it's not going to be easy to get this bird to a place where she is content with her new life. It will take a looong time and a lot of work and patience, you do realize that, right? I just don't want you to get impatient or become disappointed...