You need to lodge all year to date (YTD) amounts for every employee you employ! such as Gross, Tax withheld, Superannuation, etc.
You must lodge a valid Tax File Number for each employee.
You must assign a unique payroll number for each employee.
Business or company branch number must match the registered branch number with the ATO.
All new Single Touch Report must have a Submit report type.

You also need to report PAYG W1 and W2 for the current pay period.
W1 and W2 should not be reported as Year to Date amounts!

You must have a valid M2M credential to lodge Single Touch Payroll.

Check your computer current time, as even couple of minutes difference will fail the M2M credential connection.

Most of the errors related to M2M due to invalid authorisation, you need to make sure that your M2M credential is authorised to lodge for the current ABN or on behalf of the reported ABN for tax agents and accountants.
For non-administrators M2M, you need to make sure that the M2M account has permissions to lodge a pay event (Single Touch Payroll).

If you lodged an incorrect amounts then do not panic!, you can amend your previous lodgement by sending a full file replacement or update the incorrect amounts.

Lodging an Update report type does not update W1 and W2 amounts.
You can only send a file replacement for the last lodged STP report.

Yes you can update previous financial year STP lodgement up to 5 years.
Once the new financial year starts, you are no longer able to lodge a Submit report type for that financial year.
You must lodge an Update report type.

Each software has a unique software ID which will be used to identify the employer details, to move from one software to another without generating a duplicate payment summries for your employees, you need to perform the following:

From your current software or accountant, lodge all employees with zero amounts.
From your new software or accountant, lodge the latest payroll period as per normal.

Yes you still need to lodge Single Touch Payroll report.
The good news you can lodge quarterly through your accountant or tax agents instead of lodging every time you pay your employees.

Yes you still need to lodge all your PAYG BAS obligations as per normal and pay any outstanding taxes.
The STP is for reporting only and does not cover any physical payments which means you still need to lodge the PAYG and pay any outstanding taxes.

No it is not required for desktop software, if you are lodging via AtoConnect software, you do not need to inform the ATO in regards to Software ID. All you need is to prepare the STP report and lodge it with your M2M credential.

If you are using cloud software to lodge STP such as AccountingLink, then you need to call the ATO.