My PFL date was 24-12-2019. Till yesterday, I kept on checking my profile that may be there's a tab to upload a document and today after checking your this message, I checked my profile again and there was a "folder" in View Submitted application or upload documents, in the end naming REPLACEMENT NEEDED, CLIENT INFORMATION REQUIRED, UPLOAD FILE,23-01-2020 so i submitted my PFL reply right now through my profile and status has been changed to SUBMITTED instead of Your action is required.Try this:
Click "Check full application status"
Click "View Submitted application or upload documents"
You may find the upload option there.
If this does not work then you have to upload the document using webform.
Submitting webform will not update the Your action is required.
I also submitted the reply through IRCC Webform and IRCC Webform London.
Thank you brother,