You can plaster over drywall although the plaster needs something to adhere too such as lathe boards.
Plaster on walls is held into place by lathe boards and drywall is held onto the studs through use of nails or screws.
It would be much better to rip out the old drywall and replace the drywall with new drywall or cut out the bad portion of drywall and replace it with a new piece of drywall.
You can also repair a hole in drywall by using some plaster and some mesh behind the hold of the drywall.
To begin applying plaster over drywall, start with a clean sound surface, free of any soaps or detergents, and course with no peeling or flaking paint.
A quality finish is only as good as the quality of what is beneath it.
Then simply trowel and apply TAKCOAT evenly over the entire wall 1/16th to 1/8th inch thick.
Finish drywall to level 2 or level 3 (ASTM C840) depending on how thick your earth plaster coat will be.
Avoid sanding. If spot sanding is required carefully wipe dust with a wet sponge and allow to dry.
Prime with good quality paint primer with fine sand in it (around 10% sand).
Plaster veneer may also be applied to ordinary drywall, or over existing walls, but this requires "gluing" the existing wall surface by painting on a special adhesive compound, and then applying a thin layer of "base coat" plaster.