"He is always grumbling about something."
Translation:Il grogne toujours sur quelque chose.
It is one of the prepositions you can use in this case, as well as "à propos de/au sujet de/pour".
"Il grogne toujours à propos de quelque chose" is not accepted. (I reported it.) DL suggested the same response except with "grommeler" instead of "grogner." I probably would use "grommeler," but I hadn't seen it yet in this lesson. " (Another suggested answer used "rouspéter," an entirely new word for me! Yay!)