Hi.
I had the same error and managed to identify that it was due to several factors:

The file generated by the tagging tool did not have the required format, which is why I had to convert the file to the format specified in https://cloud.google.com/vision/automl/object-detection/docs/csv- 5 format? hl = cs. To do this, use (provisionally) a spreadsheet (TensorFlow Lite - Data Preparation Template - Google Sheets 3).
The value UNASSIGNED cannot be used. Each row must be specified as TRAINING, VALIDATION, or TEST.

Converting with a spreadsheet creates other minor issues that I fixed with a plain text editor. In the next few days I will create a Python script to convert to the indicated format.