That's unfortunate indeed, this missing document will reject the application.
I assume you've given your biometric details already? If not, then you can withdraw and get your refund. If you have done it you can give it a shot and ask HO to refund it.
You can explain due to covid you're not able to retake the IELTS because it requires travel.
"If you have already given your biometric information but are unable to travel to the UK because of coronavirus, send an email to the coronavirus immigration help centre. They will let you know if your fee will be refunded or not.
Use ‘POST BIOMETRIC REFUND REQUEST’ as the subject line.
The help centre will reply as soon as possible."
https://www.gov.uk/cancel-visa
Fingers crossed