As you are receiving the message "You may not enter transactions before the beginning of your financial year", this is most likely due to invoices or orders dated before the MYOB financial year.
You can confirm your financial year in MYOB selecting 'Setup' >> 'Company Information'.
If the financial year shows 2017, you will not be able to enter orders prior to 01/07/16.
If the financial year shows 2016, you will not be able to enter orders prior to 01/07/15.
Change/update the date in WMS order to a date before the MYOB financial year and try to sync again.