You can only add candidates to an existing Dynamic campaign manually through their mini profile, full profile or by using the Add to Campaign bulk action in your CRM.

Therefore, when attempting to add candidates to an existing campaign through their mini profile, full profile or by selecting them in your CRM, only Dynamic campaign options will appear in the list of available Campaigns. If you have launched a new campaign and you don't see it in this list, it is likely because the Campaign is not a Dynamic Campaign.

Learn more about Campaign types here.

Once a Campaign has been launched, you cannot change the Campaign type, so if your Campaign type is not the type that you need, you will need to create a new Campaign with the correct type, and copy the content across from the original Campaign.