That's not the error. We have to use @target MZ for the engine to pick up the Plugin Commands for MZ. Every one of our plugins targets MZ even if it can work in MV. It's standard practice when supporting plugins across both engines.
The screenshot taken from the Demo Project MV above was downloaded from the store so it should have the exact same configuration as yours. Its plugin also targets MZ and the functions work as intended.
Please answer the questions we asked earlier so we can troubleshoot your issue.