What do I provide as a VAT number if I am an auto-entrepreneur?

I sell used clothing on international site. I told the site that I am an auto-entrepreneur and self employed and under this regime can not deduct VAT or charge VAT therefore have not VAT number.
I provided them with my SIRET number and now received the following email. Is there another number that corresponds to the regulations below? As a business seller I have a discounted commission but it appears that I can not proceed under this status if I do not have a number.

VAT identification number / Value added tax ID
The Value Added Tax Identification Number (VAT ID no.), is a separate number that is given to entrepreneurs in addition to the tax code.
It was developed to safeguard tax revenue and is based on an IT-supported exchange of information between EU Member States for certain data.
It serves the correct application of VAT regulations in the European internal market.
Due to the missing or incorrect registered VAT identification numbers in the master data at http://www.xxxx
we ask you to check them and to correct or supplement them until 31.05.2018 if necessary.
From 01.06.2018 our system will automatically check your VAT ID in the European internal market with the central tax administration.
Only a correctly registered VAT ID at http://www.xxx allows us to calculate / charge our commissions in accordance with the VAT regulations in the “reverse charge” procedure

.