Upgrading Drupal

I have upgraded my multi-site application from 8.5.x to 8.6.x. ; have also updated the syntax of my custom profiles.

I have a few modules/themes in my custom profile; which appears to be no-more getting recognized by the application(After the upgrade) and am facing the below error : 

php-error Uncaught PHP Exception Drupal\Core\Theme\MissingThemeDependencyException: “Theme <THEME_NAME> has not been installed.” at /docroot/core/lib/Drupal/Core/Theme/ThemeInitialization.php line 122

I have github and localhost setup for my localhost Drupal 8 build.I'm git pushing to github when I make changes to my localhost build:

git push origin master

This is working correctly without errors, and I can see the updated content on my github repository when I login to github.com and view my repo.

I have already setup my cPanel server to connect to the github repository and it was working and had the base Drupal install running.

But now when I login to my cpanel via my Terminal CLI using SSH, it allows me to :

I'm new to Drupal. I would like to know how to do a minor update from Drupal 8.4.8 to Drupal 8.6.14.  I am running Drupal on my GoDaddy shared hosting service.  This Drupal site is just for learning.  The reason I need to update it is I installed some modules that look to be incompatible with a Drupal version under 8.5

PHP Fatal error:  Call to a member function getPluginId() on null in /home/mydomiain/public_html/core/modules/quickedit/src/MetadataGenerator.php on line 76

Hi, there is a white screen after updating to drupa 8.6.14, if you try to go to admin pages the tipical message "The website found an unexpected error. Please try again later." 

Thanks in advance

Recently updated to 7.65. Since then, the website is often in maintenance mode when I open it. I don't see anything in the log messages about it going to maintenance mode. How can I suss out the cause? No other users, although lots of attempts to get in.

Hello,

I'd like to move from one webspace to another while also switching from v7 to v8.

My question, is the source changed or only data read and imported into the v8?