Drupal 8.x

Hello,

I have upgraded a Drupal project from 8.5.5 to 8.7.3 or even 8.7.11 and since I have SQL problems with some Entity Reference Fields.

I have a content type with a lot of entity reference fields of File type and some of them generate SQL errors when I use autocomplete :

Uncaught PHP Exception Drupal\Core\Entity\Query\QueryException: "'' not found" at core/lib/Drupal/Core/Entity/Query/Sql/Tables.php line 367 (function ensureEntityTable).

Hi there. I'm facing some problems here when importing content to another site:

The import failed due to the following reasons:
  Unexpected error during import with operation delete for language.entity.en: Can not delete the default language

Any ideas?

Thanks for your time.

UPDATE 1

 Drupal 8.8.1 site that was installed from scratch using the new template. So core and modules are in www/web.

But two of my modules need updating, and webform requires composer to do it. But none of these work:

www]# composer update 'drupal/webform:^5.7' 'drupal/typed_data:^1.0'  --no-dev --with-dependencies
Package "drupal/webform:^5.7" listed for update is not installed. Ignoring.
Package "drupal/typed_data:^1.0" listed for update is not installed. Ignoring.
Loading composer repositories with package information
Updating dependencies

I have a sub theme, when trying to install it, I get this message: Unable to install MyTheme, MyTheme.settings already exists in active configuration. This sub theme was previously working, so I do not believe this is a misconfiguration or error in theme build. The theme is now not installed. 

I'm on drupal 8.8.1 / nginx/1.8.1 / php 7.3.12.

There is mytheme.settings.yaml file in the config/install directory, but I suspect this is not a file problem but rather a problem inside drupal database.

I have a site on Drupal 8 and Commerce 2.16 with a marketplace (several merchants with several stores).

Each merchant can fill in the SKU field with their product variations. The problem is that the SKU must be unique. How can I insert a code at the start of the SKU ? For example the parent product ID.

In my case the product is created without variation and the variations must be added afterwards.

Here is an example :
[product id]-[merchant sku]

The [product id] part should be automatically filled in and locked.

So I began learning drupal a couple of months ago and have mostly been playing around with it while trying to absorb all of the knowledge I can. I have my first project in the works but I'm wondering how to actually go about its construction.