I installed the delta theme, but when I install the plug in car attribute it tells me the following error "Error importSQL::ModelCars cars_attributes/struct.sql", I wanted to understand, when you buy a plug in or a theme can you install it on multiple sites? Or should I buy another one?

In plugin review:

From technical point of view, everything is OK in plugin. I recommend you to take SQL from struct.sql and run it on your database. Usually there is issue with foreign key:
FOREIGN KEY (fk_i_item_id) REFERENCES /*TABLE_PREFIX*/t_item (pk_i_id) ON DELETE CASCADE ON UPDATE CASCADE,

Further investigation shown that this problem most frequently happen when your database tables use MyISAM engine.

Osclass use InnoDB database engine by default.
Foreign key between table in InnoDB and MyISAM table cannot be created.
Make sure all your tables use InnoDB engine.

Then install plugin again.

More details: https://forums.osclasspoint.com/osclass/foreign-key-plugintheme-installation-error/