When contacts in a sequence are set to Inactive, it means that they are missing the data for the variable you used in the Steps templates.
The variable pulls the data from a prospect's profile and, if the field connected to this variable is empty, the system will mark the contact inactive and block them from receiving the sequence.
You can check which variables are missing in sequence > Preview tab.
For your prospects to become Active, just add the missing data to the prospect profiles (or remove the variables from the templates), select all people with the Invalid status > Status > Active > Set status and your sequence will start sending.
