This error occurs when a module tries to call a method that existed in older Magento 2 versions but was removed in a newer version that you have upgraded to (in this case Magento 2.4.6).
Specifically, the method `loadByCustomer` used to exist in the `Magento\Framework\Model\ResourceModel\Db\AbstractDb` abstract class but was removed in later versions.
To fix this, the module needs to be updated to no longer reference this removed method. Usually the solution would be:
1. Identify what custom or third party module is producing this fatal error. Check the stack trace to pinpoint the origin.
2. Open the module's files and find where `loadByCustomer` is being called.
3. Remove the references to the non-existent method. The business logic may need re-implementation using valid 2.4.6 methods.
4. Upgrade the module to a version supported on Magento 2.4.6 if available.
Essentially the module containing this method call should be patched, refactored or replaced with a 2.4.6 compatible version that implements the equivalent business functionality through alternative valid means in the newer Magento release.
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