Error installing Theme on GoDaddy Drupal 8.4.5
Based on the two errors you noted:


"The website encountered an unexpected error. please try again later."
the file is over 2mb


It sounds like your web-hosting provider/CPanel account has a limit on file uploads set, and: 

a) your Drupal Core update/upgrade wasn't completed successfully

b) you may not have a temporary directory configured properly for unzipping the theme archive.



Recommended course of action would be to:


finish your Drupal 8.4.5 upgrade (or perform a fresh install on GoDaddy).  If you're drupal core codebase was updated, be sure to go to update.php to ensure all Drupal database updates were applied.
Once Drupal Core has been updated and you can navigate the site/add test content successfully, unzip the theme codebase locally and upload the "nexus" folder to your Drupal site's /themes/custom/ folder using either GoDaddy's file manager, ftp, or via version control tools such as git.
With the theme folder manually installed to the server, flush the cache of your Drupal site; go to Appearance, select and enable the Nexus theme.