Please follow below step to solve this issue.
1. Login to your Plesk WebUI.
2. Go to websites and domains and select the respective domain that you are facing the issue.
3. Click on PHP settings
4. Browse to the bottom of the page and you will seeĀ Additional directives. You can specify inside the box the extension that you want to enable by key in as per below.
extension=mongodb.so
5. Once done, click Apply to confirmed changes.