The Google Chrome team has been randomly rolling out a new setting in Chrome version 80 and above which can cause an Invalid Shop message to appear even when the correct Shop Name is being used with third-party applications. PayJunction's engineers are working to make an update to prevent this error.
If you cannot use a different browser while this issue is ongoing, you can follow the steps below to manually disable the setting.
How to Fix:
Open a new tab in Chrome
Paste the following into the location bar: chrome://flags/#same-site-by-default-cookies
Find SameSite by default cookies on this page and set it to Disabled
Click the Relaunch button that will appear at the bottom of the page to quit and reopen Chrome.
Some users might need to reboot their computer after making this update to get Chrome working again