Why would there be a check mark under credit for other dependents instead of under child tax credit? At the time of filing, we had one dependent (approximately 6 months)

They have to be under 17 on Dec 31 to qualify.  And be your own child.  Is this for a new baby?  A child born (even on Dec 31) is considered to live with you the whole year.  So be sure to pick the Whole Year at the top of the drop down box.

"Approximately 6 months" is not good enough. For 2020, the child must have lived with you for MORE than half the year.  In the TurboTax (TT) interview, when it asks how long the child live with you, you must answer 7 months (or more) to get the child tax credit. The child must be a "qualifying child" dependent, not a "qualifying relative", as well as being under 17. 

There are two types of dependents, "Qualifying Children"(QC) and standard ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, student status, a relationship test and residence test (more than half the year).. Only a QC qualifies a taxpayer for the Earned Income Credit and the Child Tax Credit. They are interrelated but the rules are different for each.

The support test is different for each type. The support test, for a QC, is only that the child didn't provide more than half his own support. The support test for a Qualifying Relative is that the taxpayer provided more than half the relative's support.

See full dependent rules at: https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Return/INF12139.html