Has she got a light in her room? This is prime age for scared of the dark to start.
I'd sit with her. For this to work she needs to trust you'll stay until asleep. If you seek out before she's asleep she will start fighting to stay awake as a way to jerp you with her. Whereas you can remove that battle by staying until asleep. It's reasonable to expect ger to go to sleep with 10-15 minutes if she trusts you'll stay.
Then it's about a process of always giving enough reassurance, but in an ever withdrawing way.
So start sitting by the cot, firm hand on her chest/back and stay until asleep. Then start removing you hand when calm, hand back if distressed and withdrawn again once settled. Stay until asleep.
Then firm hand in chest/back to settle, stand and turn away from cot when settled. But stay right there all the time until asleep. Turn back round and settle with firm hand when distressed, turn back and wait when calm.
Then settle and step away from cot. Return if distressed, withdraw when calm. Stay until asleep.
Then settle and wait by doorway. Return if needed, withdraw when calm. Go back as many times as needed. Stay by door until asleep.
Then pop back and forth from doorway at sleep time. Then milk around upstairs, but not by door. Then close door but be upstairs until asleep to return quickly if called.
Then close door and leave...