Parameter must be an empty string or follow the format: rgb(###, ###, ###)
The error is related to one of the Rich text field tagged in the document that contains a colour parameter which is not supported by DocGen.
We do not have visibility of what exact field or part of the text is causing the issue. If your users are copying text from the internet, it might contain background colour which is not visible. The issue cannot be resolved by us as the error is simply that we are receiving bad data that cannot be processed into the document as a text.
Solution to strip the unnecessary formatting might be to use a Long text field instead or strip formatting before pasting it into the rich text field (you can use Ctrl+Shift+V to speed up the process).