Check that both parcels of land are adjacent (they must share a border at some point).
Check that both parcels are within the same region. You cannot join land across a region boundary.
Check that both parcels have the same owner. You cannot, for example, join your group land to your personal land.
Next, right-click the ground and choose Edit Terrain or hit Ctrl-5.
Now you are ready to create the yellow bounding box. Left-click and hold your mouse down, and drag the yellow selection such that it spans the border between the two parcels you want to join (see the image below). You need to have selected a little land from both parcels that are to be joined. You do not need to select all of both parcels, just some from both.
With that done, you should now be able to click the Join button under Modify Terrain in the Edit Terrain menu.
. With both parcels now joined into one, check the About Land window. Firestorm users click World – Parcel Details, or simply right-click the ground and choose “About Land” and make sure that your settings are correct.
Hope this is useful
Preferred land Management