Aiya, at 15 months the crying and screaming is totally normal, she just needs time to adjust to the new sleep space. Try the Ferber method: put her in the cot, walk out, then if she cries, go back in after a few minutes to give a quick pat and reassurance (less than 15 seconds!), then leave again, you gradually stretch the waiting time to 7 minutes, then 10. My neighbour tried this with her daughter around the same age and said the first two nights were rough but by night four the baby was settling on her own. The crying does reduce quite fast once they get used to it, so just stay consistent and don't give in halfway, that's the part that makes it take longer.