A machine credential is valid for 2 years. Please see the ATO website: How to install a machine credential, for information on how to set up and install a machine credential.
If Software Required still displays in your browser after downloading your browser extension, review this article How do I install/enable a browser extension to create a machine credential?
If you are using HandiSoft Online, refer to the article How to save the Machine Credential keystore file.
Note: If you have recently updated your Tax Agent Details or ABN, all users will require a new machine credential.