Hi, I have doubts if he is for sure "professional" otherwise he would use alternative method of template installation. Or maybe he just forgot about those steps.

1. Check if you can install any other template, for example, use Helix Ultimate template package.
2. Then download Quickstart package and upload folder /template/shaper_indigo from it
3. In Joomla Admin: Extensions - Discover - Click Discover button - Install found template
4. Check if Helix Ultimate plugin is enabled from step (1)
5. Later check System - System Information - Folder Permissions

Ok helix template wont upload either, error message below

JInstaller: :Install: Can't find Joomla XML setup file. The Joomla package cannot be installed through the Extension Manager. Please use the Joomla! Update component to update Joomla.

So now you're 100% sure that - it's not our issue ! Many topics with answers are here: forum.joomla.org

BTW

In Joomla admin: check System - System Information - Folder Permissions . My role ends here.

Not sure how to do this, I am not sure why it is so difficult to install, I have never had this problem before. Perhaps I should just get a refund and find a template elsewhere?

Maybe download and try to install free template from different developer - probably you would get this same issue.

BTW Are you able to install any/random extension?

We are not Joomla support. I told you to search on offcial joomla forum, there are tons of similar topics with answers.

BTW

Every template can be installed manually as well. Using FTP tool. It takes only 3minutes more. If hired by you developer cannot do that, I can.