Unable to add feature service definition Error code:400

[MichaelRuffino]

I'm trying to publish a survey and I keep receiving "Error code:400  Unable to add feature service definition" . From what I can tell the form doesn't have any errors at this point, and I can't seem to figure out why this doesn't want to publish. I have published the same survey with a different file name, but it no longer exists, neither does the folder that was created in AGOL . It actually creates the feature layer, but not the form in AGOL. Any assistance would greatly appreciated. Thanks  

Version 2.6.4

[TL2]

There are three "tank_mfr_other" field names?

Also, all field names should start with a lowercase.

[MichaelRuffino]

Thanks T L.  We cleared the "tank_mfr_other" issue up and the all of the upper cases, but it still didn't solve the problem. After getting another set of eyes on the form, we realized there were three "located_in_test" field names which we cleared up and was able to publish with problems

[deleted-user-BB7WEi0B7-KA]

I had this issue. I found I had duplicate field names where I was pulling data to Lat, Long fields from photos. Changing these  to unique names allowed me to upload/publish. Probably best to Create > New Survey and point it to your xlsform spreadsheet. This will update all the "types" and data validation on the survey sheet. This should highlight duplicate field names.