Upgrading Drupal

Hi there

I'm still getting used to the update process on drupal 8, but had managed to do it a few times last year using composer, running on my dedicated server with php 7.1

I am really stuck with upgrading my site now, from drupal 8.7.5 to 8.8.3

I found this article - https://www.palantir.net/blog/guide-drupal-8-8-update and followed all the steps but some error messages came up

Your requirements could not be resolved to an installable set of packages.

I have inherited several Drupal 8 sites that are in version 8.6.3. No updates of any kind have been done since the sites were built. So no updates have been done in over 2 years.

The sites were originally done as Drupal 6 sites and then were migrated up to Drupal 8.6.3. The db is full of legacy tables that were carried over, no big deal except this bloats the db making it impossible to work on in the server and in docker where I cannot set the max_vars_input in the php settings. 

I am now working on this on a local MAMP pro environment.

I've upgraded 2 sites now and I'm getting the same errors on both sites.

Notice: Undefined index: status in _update_requirement_check() (line 117 of /web/core/modules/update/update.install)

I just upgraded Core to 8.8.3 from 8.8.2. My site language is German. Now I get the message that translation-files for English are missing for 70 projects, eg. 

Going from 8.8.2 to 8.8.3, I did:

For those of us just starting out with Drupal, would it be possible to use the D9 alfa and get the Olivero theme Even though still in the final stages of development? With only a few months to go before the official launch for those of us working on creating a new website this would seem the logical course to take. Perhaps something similar to the Windows insider program. If the new theme is as great as it seems to be, this would avoid having to transition and save time. As opposed to learning one system that will be short lived and then switch to something new.   Thanks