This error message means that this prospect has already gone through this sequence. If you still want to put the prospect in this sequence, you must do one of the following:
Option 1: Remove the prospect from the sequence. This will not affect your sequence's statistics or alter your prospect's history. This bypasses the sequence's safety settings to allow the prospect back into the sequence. To do this go to your Sequence >> Sequence Settings >> Select Prospect >> Gear Wheel >> Remove >> Confirm.
Option 2: Change your sequence's Safety Settings. However, this will allow all prospects to be added to the sequence multiple times, so keep this in mind when adding prospects to this sequence. To do this go to your Sequence >> Sequence Settings >> Edit Ruleset >> Change Safety Setting (Prospect can be added to this sequence...:) >> Save.
Once you have completed one of these 2 options, you will be able to add the prospect into this sequence!