You're trying to create a new custom field using the same Identifier as a custom field that you have used before and transported it to Productive system but now it's deleted. The error message below shows up when you try to save the new custom field:
Name YY1_Field_name is not valid.
It was already transported into another system
SAP Marketing Cloud
SAP Marketing On-Prem Edition
custom field, field name is not valid, transported , KBA , CEC-MKT-EXT , Marketing extensibility , Known Error
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