To install Live Assist for Dynamics 365 the user which starts the installation will require the System Administrator role, as per this article: Installation Guide.
To do this you can log your user in to Dynamics 365. When selected one of your application as seen below, this case a Custom App, click the cog-wheel at upper-right of the page then select Advanced Settings.
Note: if your screen looks different from the above, delete the part of the URL after ".com", and replace it with "/main.aspx" (e.g. https://acmeusa.crm.dynamics.com/main.aspx).
Go to the Settings menu and select Security:
Go To Users:
Select your admin user from the list of Enabled Users and go to Manage Roles
Inside Manage User Roles ensure that System Administrator is checked, then save your changes.
Note: Enabling Live Assist Roles in this area is not supported.
Occasionally promoting a user to admin will fail with the following error.
This can happen for newly created Dynamics organizations or users. If you do see this issue and all the other settings for the user look good then wait for 10 mins or so and try again ... typically the operation will then be successful.