Below are the two web addresses used for Single Touch Payroll in Lightning Payroll.
ATO SBR2 Cloud shows current server status of their main services. PAYEVNT.0004 is the STP service. If it is red, the service is down. Please try again later.
https://sbr2.status.ato.gov.au/
The other service required to send STP is the STS/Machine Credential Secure Token Service. Below is an example of how a healthy page would look.
https://softwareauthorisations.ato.gov.au/R3.0/S007v1.2/service.svc