I am using Lightsail only. No other amazon services. Why i get charged in AWS Key management service. What it is?

Hello there, thank you for your question. Pricing for AWS Key Management System is available at: https://aws.amazon.com/kms/pricing/ . You refer to the API requests, which as per your data fall into the free tier. However, as is stated on the pricing page:

Each AWS KMS key that you create in KMS costs $1/month. ... If you enable automatic key rotation, each newly generated backing key costs an additional $1/month.

So you are charged for more than only the API requests. You can find pricing examples at the bottom of the page, which can provide some more insight in the calculation. You can also use the AWS Calculator (https://calculator.aws/#/createCalculator/KMS) to determine your monthly cost if you know all the data.

Contact AWS customer service ... it's probably a case where you were playing around and inadvertently activated the key management service.\

I had a similar problem with a directory service getting enabled. I contacted customer service and they walked me through disabling the service and credited my account for the few months that it was active.