I had this same problem trying to upload the T4 data as an .XML file via Represent a Client. I downloaded a free XML editor online (MS XLM Notepad) and was then able to edit the file to add in my RepID.
I did this by duplicating the TransmitterAccountNumber folder and then editing it using the same form outlined on the CRA site:
You can view the data by opening the XML file in MS Edge.
Here's how it looked at the top of the XML file BEFORE I edited it (BN and RepID changed to all Xs):
<T619>
<sbmt_ref_id>XXXXXXXX</sbmt_ref_id>
<bn15>XXXXXXXXXRP0001</bn15>
</TransmitterAccountNumber>
<summ_cnt>1</summ_cnt>
<lang_cd>E</lang_cd>
<TransmitterName>
</TransmitterName>
<TransmitterCountryCode>CAN</TransmitterCountryCode>
<CNTC>
Here's how it looked at the top of the XML file AFTER I edited it (BN and RepID changed to all Xs):
<T619>
<sbmt_ref_id>XXXXXXXX</sbmt_ref_id>
<TransmitterAccountNumber>
<bn15>XXXXXXXXXRP0001</bn15>
</TransmitterAccountNumber>
<TransmitterRepID>
</TransmitterRepID>
<summ_cnt>1</summ_cnt>
<lang_cd>E</lang_cd>
<TransmitterName>
</TransmitterName>
<TransmitterCountryCode>CAN</TransmitterCountryCode>
<CNTC>
It probably sounds more complicated than it is. It's just downloading an XML editor, duplicating the Transmitter Account Number folder, changing its name to "TransmitterRepID", then changing the subfolder name to "RepID" and changing the text from the business number to your CRA RepID. Hit save and submit, you won't get the error anymore. Took 5 minutes start to finish.
