Firstly, please check again this issue from github:

We already provided solutions for this issue.

If you did follow but still have the error. Please follow these steps:

- Backup your store.

- Turn off any cache managers that are activating in your server such as Redis, Varnish, CloudFlare, Op-cache.

This information you can ask your host provider.

1. Run this command: rm -rf var/cache/* var/generation/* var/di/* (generated/* with Magento 2.2 ~)

2. Turn off Core Module by running module:disable Mageplaza_Core command.

3. Delete app/code/Mageplaza/Core folder

4. Run composer update again to get back the module_core (which inside public_html/vendor/mageplaza/module-core)

5. NO Core folder inside app/code/Mageplaza/ because of step 3.

6. Turn on Core module if needed (due to step 2).

Turn on the cache that you disabled.