Drupal 8.x

I'm an amateur, not a professional developer, but I've been using Drupal for a number of years and have built a number of D7 and D8 sites, but this is brand new for me and I'm baffled. I'm not even sure this is a drupal problem as opposed to something related to my host.

Created new MySQL database, uploaded tar.gz of 8.8.6, unpacked it, navigated with my browser to install and did the install process as usual. After the last step in that process (setting basic site configureation with site name, email. admin name and password) it get the following message:

Hi everyone.  I'm new here and have a question for my drupal 8 website.  I'm trying to embed some code and am having a little trouble...

Hello,

I have the MAYO theme installed am using a subMayo creation. According to the MAYO Theme write-up, it does everything that I want it to do. The problem I have is HOW? Is there an interface that I'm missing somewhere? Do I need to make all of my changes in the .css files? Is there a documentation file somewhere that actually has some information?

At this point, I just want to change the header image (keeping the logo.png image intact) and create a site-wide background image.

Any assistance would be greatly appreciated,

I'm running 8.8.2 (yes, I'll be upgrading soon), and I'm needing to run a small PHP snippet in a block to fetch the output of a command on my Linux web server. I would normally use exec(), but this function is not working in my version of Drupal. I really don't want to have to make my own plugin/module just for a simple code snippet. Any ideas on how I can do this simply and easily? 

Is there a module, other than the Webform, where we can change the Contact form's fields?

example:
Change Your Name to First and Last Name
Change Your Email to Email
Add Company Name
Add Phone#

I did search and couldn't find any solutions yet.

Also am surprised that the default form is not letting us change those fields.

Drupal 8. Multiple sites sharing a single codebase.  One .htaccess file for all.

I am receiving the same "page not found error" across all sites. 

For example, someone tries to visit https:domain1/wp-admin/admin-ajax.php and https:domain2/wp-admin/admin-ajax.php ... Different domain names, but always the same URL strings.

Other URL strings include /phpmyadmin/scripts/setup.php and /1/wp-includes/wlwmanifest.xml and so on.