Craft 3 Upddate: Plugin Errors I have tried to get a number of plugins from the Plugin Store but each time on install, I get the following errors: 

Status: Not Found

Response: {"error":"Template not found: internal_error.html"}

Has anyone seen this as well?
internal_error.html isn't a file that is shipped with Craft.

Most likely what is happening is that you're running into some fatal PHP error (like bumping into your php.ini file's memory_limit or max_execution_time setting) and your host has configured all fatal errors (or anything with a 500 Internal Server Error response code) to redirect to their internal_error.html template.

Craft doesn't know how to route to that template, so you get the error above.

I'd check Craft's logs at craft/storage/logs and/or your web server's error logs for the real underlying error.