You probably didn't install the Core module or missed some commands, please follow our installation guide here: https://www.mageplaza.com/install-magento-2-extension/. We provide both composer and non-composer methods.
Please notice that we listed all the steps so kindly help us read it carefully.
Our Core module is updated frequently so make sure that you are using the latest version of it.
Here is the repository https://github.com/mageplaza/module-core
Other messages that indicate missing Core module are:
- Mageplaza\Core\Helper\AbstractData does not exist.
- Class Mageplaza\<extension_name>\Helper\Data does not exist.
- Specified invalid parent id (Mageplaza_Core::menu)
- Call to undefined method Mageplaza\\PdfInvoice\\Helper\\Config::jsonEncode