Hi there LindyMH
That error typically indicates there are invalid characters in the TPAR file. This can be caused by manually changing the TPAR file or invalid characters entered into Supplier cards. You can check the supplier cards for invalid characters entered into the address or company name fields. Also make sure the file isn't being emailed or copy/pasted to a USB etc. as this can change the file size. Once done, you can download the file again and upload it to ATO. You can also provide us with the diagnostic log to find what's causing the error. Let me know how it goes.
Best regards,
Doreen