Customer's question:

I get an error "The plugin does not have a valid header" while installing TerraClassic WordPress plugin. How should I proceed?

Answer:

Please note that PE TerraClassic is a WordPress theme NOT a plugin, and if you want to install the demo copy of this theme you need to follow this tutorial for installing the WordPress theme or the whole demo content - depending on what you need to do.

Probably you are trying to install TerraClassifieds WordPress plugin. Find the documentation for TerraClassifieds here.