Actually, UNA can run on shared hosting. The problem is whether your web hosting provider meets the requirements to run UNA, which can be found HERE:
On some shared hosting providers, what happens is they host multiple websites on a single server. So let's say there are 25 websites (some do a lot more) on a single server. If the provider has configured that server with PHP version 7.1 and configured with certain functions/modules enabled then, ALL 25 websites are subject to the same configurations. If a change is made, then it reflects everyone and not just your site.
As Baloo mentioned above, some hosting providers will allow you to over-ride the PHP by creating your own php.ini file and placing it in your root directory. But that doesn't always work.
Some hosting companies will assist with "special" requirements however, they mostly tell you to upgrade to a VM or dedicated server if you need more resources or requirements.
So if all else fails and you have recently purchased shared hosting from a 3rd party, some will give a refund and you could find a new hosting providing that does meet the requirements.