SOS MONEY ISSUES

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I would like to address a question about the VAT number of the client. In my country I should declare the VAT number of my clients. What can I do if the client

I have tried to put my greek VAT number but it says it is invalid??? My ΦΠΑ είναι *** Best regards Iia Doxastaki Mesorachi Kentri Ierapetra Greece