this appears because of so many reasons , its impossible to fix usually unles your the Terrain maker .
Known reasons to me are :
by ensuring you have all the CA and subfolders folders unbinned correctly on your pdrive and Config cpp`s unbinned , in addition
and this is the major fix , your Required addons section in cfgpatches in your terrain .cpp has the correct classes listed .
In addtion you can write a new config for the p3d in your terrain.cpp by adding a cfgvehicles section andthis will work (not tried for BIS binned p3ds but works for mlods being made by terrain user).
One day i hope to sytematically take notes , but as always these things take so damn long to figure out , your way behind and simply want to move on.
I always believed that some models are simply not configged as anything , i dont know how BIS escape this error , best way is to get an all in one config and search the p3d name that erroroing , if its not there then make a class for it, if it is there ,find the cfgpatch header for which the p3d resides and add this to your required addons.