I am facing the same error code: PAB.BR.0020.
Today I contacted ATO for linking code (took me more than 2 hours in total... phew). They provided me several linking code but nothing worked. Every time it showed:
Linking code cannot be found or has expired. To continue you need a new linking code. Error code: PAB.BR.0020.
I told the ATO support about the error code, they said it may be a problem with mygov and I need to contact them. I do not know what I should do at this situation. I would appreciate if anyone can help me.