When creating 'Collect if' conditions within group rows, use the cycle specifier this to refer to question values within the same row. See image
If you only want to enable the 'Stop date' question in each row if the medication is completed (the MedComplete question) you could use the following 'Collect if' condition:
MedComplete(this) = 'c'
Note that 'c' is the category code for 'completed'.
To disable the start date field in each row until a medication has been entered, use the following 'Collect if' condition:
isknown( MedName(this) )