Hello I want to use only one server can you help me?

Hello.

As described in the following document, Lightsail charges only for what you use.
So even if you stop Lightsail in the middle of the month, you will still have to pay for what you have used up to that point.
If you have already deleted Lightsail in the target region for which you have already been charged, you should not be charged any more.
If billing occurs, check to see if there are any snapshots or Elastic IPs left.
https://aws.amazon.com/lightsail/faq/?nc1=h_ls

Lightsail plans are billed on an on-demand hourly rate, so you pay only for what you use. For every Lightsail plan you use, we charge you the fixed hourly price, up to the maximum monthly plan cost. The least expensive Lightsail plan starts at $0.0047 USD/hour ($3.50 USD/month). Lightsail plans that include a Windows Server license start at $0.01075 USD/hour ($8 USD/month).

"I just want to use a single instance connected to the Korea server. Am I currently configuring it this way? Could you please confirm if only the Korea server is set to incur charges?"

Lightsail is not activated in Korea because the region being billed is not the Korea region.
Select ap-northeast-2 to start in Korea.
https://lightsail.aws.amazon.com/ls/docs/en_us/articles/understanding-regions-and-availability-zones-in-amazon-lightsail