1. The obvious answer might be that your API token is actually missing. Check your Request Headers if you are using the Platform API.
2. However, you might be getting the error if you're using the client SDKs. In this case, check if you have connected to SendBird. In many cases, this error is caused by not connecting at all, or calling another SendBird method immediately after invoking the connect() method. In the latter case, you must wait until you have received a callback from connect() before you are completely connected!