I am currently playing on the "Medieval Minecraft" fabric modpack, 1.18.1.
I tried taming a Megaparrot but I can't get the saddle on them, both in inventory, or shift right clicking them.

Is there anything I can do to fix this?

If anyone has the same issue, you should check if your parrot is not still growing up.

I though I had this issue and noticed with Jade that there was a timer. I tried with another parrot that didn't have the timer and I could give him a saddle :)

Huh.. I wonder how they got a growing timer. They should be full grown at all times. Weird. thanks for reporting @ChielTheOread and thanks for (kind of) a temp fix @Bob74