I haven't received the v-bucks in the December promotion of 2018

I didn't have gold before that date

you'll have to go through support.xbox.com/contact-us as nobody has access or ability to do anything with your account, here on the forums.

If memory serves me correct, you had to have no Xbox Live Gold from around October 10th 2018. If you had an active Xbox Live Gold subscription after that cut off date, you were not eligible for the V-Bucks offer.

The promotion was for new accounts or accounts with no history from a certain date as mentioned.

Actually you could have previously had gold, just not after a certain date which was 10/01/2018

Terms and conditions:

Valid only for new subscribers or users who canceled their subscription before 10/1/2018. Limit: 1 offer per account; credit card required. Only valid through link in this offer, in all Xbox Game Pass markets excluding
Quebec. After first month, subscription continues to be charged at the then-current regular monthly price (subject to change), unless cancelled. Plus applicable taxes. Game catalog varies over time. Not valid for existing subscribers . After promotional period,
subscription continues to be charged at the then-current regular monthly price (subject to change), unless cancelled. Plus applicable taxes. Limit: 1 offer per account. Only valid through link in this offer, in all Xbox Live markets excluding Quebec. Credit
card required.

The promotion was for new accounts or accounts with no history from a certain date as mentioned. You may not have been eligible. You should contact whoever told you that you were and have them look into things for you.