Unfortunately you cannot install sample data in a substore without wiping the entire data of all stores. This is a limitation of OpenCart since all store information (categories, products etc.) is stored in a single database.
If both stores are new and there is no valuable information, you can do as we’ve advised you in our previous replay – uninstall and again install Trendo theme (module). Once you get back the message dialog there will be an option to choose which theme to be installed to each of the stores (main and substore).