I am unable to choose my dependent for the dependent care credit?

In order for the dependent care credit to populate, she would need to be considered disabled.  You can indicate this in the interview about your mother in TurboTax Online- navigate to My Info, Your household, select your mother. Continue through the interview and on the screen "Less common situations" check the box for "was disabled." 

For tax purposes, the definition of disabled is: the person can't engage in any substantial, gainful activity because of a physical or mental condition and a doctor determines that the condition has lasted, or can be expected to last continuously, for at least a year or can lead to death.