"Card_InvalidABN

This error happens when the system tries to sync a customer or a supplier to MYOB and the entry for ABN is incorrect. Simply find the customer or supplier (you can check the document that it is trying to sync and identify them) fix the ABN and try again,

E.g

Error Syncing Bill #SRV-59351:  - : 
{
  "Errors": [
    {
      "Name": "ValidationError",
      "Message": "Card_InvalidABN",
      "AdditionalDetails": "Number:(ABNVO)",
      "ErrorCode": 2530,
      "Severity": "Error",
      "LearnMore": null
    }
  ],
  "Information": "Warning, error messages have not been finalised in this release and may change"

What we'll do is find bill SRV-59351, then open the bill's supplier, check the ABN, fix it then saved. then try and sync again.