and to add to the response, the rules are simply different.
The Health Care laws permit children to remain on your plan until they attain the age of 26 while the tax credit rules change at various ages
Per the link below from TT, if the child is over 23 on 12/31/18 then the income test come into play. The fact that TT asked you the income question would suggest she is over 23.
The income question is not asked if the child is under 24 as that doesn't determine your child's eligibility for either the $2000 child tax credit (under 17) or the $500 other dependent credit (under 24)
https://turbotax.intuit.com/tax-tips/family/rules-for-claiming-a-dependent-on-your-tax-return/L8LODb...
