MSIX .wapproj service

In all likelihood your service wants to start under the SystemServices logon account.  In that case, there are two capabilities that must be added to the manifest:

<rescap:Capability Name="localSystemServices"/>

<rescap:Capability Name="packagedServices"/>

Depending on the scenario, you could also consider rewriting the service as "out of process COM" instead of a service.

• Services are available for interaction to software outside of the MSIX container, and have system privileges to perform actions on behalf of the user without UAC prompts.
• Out-of-Process COM supports calls from inside the package that can perform system privileges to perform actions on behalf of the user without UAC prompts.

While I wouldn't re-write without a reason, there are some situations in an MSIX package where when the service doesn't cut it, the out of process COM can.  Most likely, you just need the second capability for localSystemServices.