Child custodian - Turbotax saying I am not the custodian

A little help please-- I am married but filing separately this year.  I entered my son as a dependent this way: 
 How many months stayed with you:  7   (more than half the days of year) 
 Did he pay more than 50% of own expenses:  No 
 Do you have a custody agreement?  No. 
Let us know which parent he spent the most nights with in 2020:  (Yes) Me.   
Has his other parent waived their legal right to you so you can claim him as a dependent on your return this year (yes/no)?   
I entered "No" - and Turbotax responded  "It turns out he doesn't qualify as your dependent".  What??? 
As stayed with me more than half the year, I am the custodian from the IRS' perspective, and should not need a legal right to be released to me.   
Why does TurboTax say he does not quality as a dependent?  How else should I proceed?  
We could fill out a 8332 but the 8332 is for the IRS-determined custodian (me) to release a claim to exemption to the non-custodian - -  Not for the non-custodian to release claim to the custodian. Or is it?