Hello! I received a review of an article using data from PISA, PIAAC studies, where I used an analysis of average values, I did not use weights:

"The authors did not take into account the specifics of working with plausible values ​​(the estimates cannot be averaged directly, standard errors must be adjusted, as described in the PISA manual https://www.oecd.org/en/about/programmes/pisa/how-to-prepare-and-analyse-the-pisa-database.html). The authors use non-standardized PISA and PIAAC scores for comparison between countries, which does not allow us to talk about the magnitude of all the differences described."

I don't understand what is meant and how can I do this? The manual is not clear to me.