Upgrading Drupal

I'm getting this failure, and have been unable to find a solution.

Update make_menu_link_content_revisionable

• Custom menu links have been converted to be revisionable.
• Failed: Drupal\Core\Entity\EntityStorageException: Exception thrown while performing a schema update. Cannot rename tmp_46f0cbmenu_link_content_r__133351678b to menu_link_content_r__133351678b: table menu_link_content_r__133351678b already exists. in Drupal\Core\Entity\Sql\SqlContentEntityStorage->wrapSchemaException() 

I upgraded the version from 8.3.7 to 8.7.2 in our UAT server.Once upgrade the version I opened the login page, I am getting the below error.

The website encountered an unexpected error. Please try again later.UnexpectedValueException: External URLs do not have an internal route name. in Drupal\Core\Url->getRouteName() (line 555 of core/lib/Drupal/Core/Url.php).

I've just installed Drupal 8.6.2 which was with my hosting package, on a new website, and am notified of a core security update to install 8.7.5.

It says I have to install manually.  Years ago, when set up a site, I had ftp details, so I could upload files.  I didn't get that this time (different company), just login and pwd for the "database", with a path to the name of the database name.  Presumably this is the MySQL darabase?

What should I look for in order to find ftp details?

I have run some updates on a site, and Drupal 7.67 seems to be the latest D7 release, and supposed to be still supported until 2021 from what I read, but I get the following image:

(OK, I can't add an image, the image add text helpfully tells me 'must be hosted on Drupal.org, upload with “Add a new file"'.  But I have no "Add a new file" facility available.....

Anyway the image is of the /admin/reports/updates/update url on the site, and it includes the following text:

Manual updates required

Updates of Drupal core are not supported at this time.

I am having a variety of problems with attempting to move a D7 site to D8, some of which involves trying to use Acquia Dev Desktop as part of that process. But before getting into the details of those problems, I want to ask a bigger question that is directed to those who have significant experience with this type of migration, specifically to Drupal 8, taking these points into account:

I am trying to upgrade the version of drupal/acquia_connector running on my site. The trouble is, when I try to upgrade the module with a composer update, composer throws an error and refuses to do the update. I'm hoping someone could offer some help with either:

getting the upgrade itself to complete, or
at least understanding the composer error message in hopes of further diagnosis.
Here are some details.

The site is currently running: