Did I not get the sixth child credit on the 15th because I owe/defered social security payment that is due the 31st?

If you did not get the ADVANCE CTC payments then you will get the entire credit you are eligible for  on the 2021 return ... follow the screens in that section carefully ... when it asks how much you got in advance answer it truthfully.  If you are married then there will be 2 boxes ... if you got one combined payment then either put it all on one person or split the payments in 1/2. 

The link below to an article will provide some clarity on the credit for 2020 & 2021.  Also, the article does provide some info on the Other Dependent Credit.  Hope this helps.

https://turbotax.intuit.com/tax-tips/family/child-tax-credit/L9ZIjdlZz

There is no place on the IRS forms or in TurboTax (TT) to "populate the remaining (child tax credit) that you are entitled to".

Instead, you get the full Child tax credit ($3000 per child, $3600 for children under 6), but the amount you got, in advance is subtracted, on Schedule 8812, before the net amount is entered on lines 19 and 28 of form 1040.