Magento2.2.5 How to access Magento Log file for Web API?

Just run the below command in magento root directory, it will show that log file location.

grep -r "Report ID: webapi-5c644d10f0e26" var/

Note: Change the above report id as per your error report - e.g) Report ID: webapi-xxxxxxxxxxx

Up to my mind it is in exception.log file and find the line with the content webapi-5c644d10f0e26 for the detailed report.