Consider this app.config appSetting entry:
The best place for services to write their logs is %PROGRAMDATA% which, by default, is in C:\ProgramData\.
Try:
<add key="serilog:write-to:RollingFile.pathFormat"
value="%PROGRAMDATA%\ServerService\Logs\log-{Date}.txt" />
(Program Files is usually considered to be read-only, and writing stuff here will lead to oddities being left behind unexpectedly during uninstall.)