If you are facing an error inside your sequence, regarding contacts in a sequence that are falling under the 'Issues' tab, this would occur if those contacts do not have a contact owner assigned and your 'Send From' settings inside the sequence are set to send from contact owner.
Once you have assigned the contacts a contact owner, please note, they will not immediately fall out of the issues tab, and instead, will fall out as they get enrolled into the sequence.
If your sequence hasn't begun yet, to clear up the Issues, we would advise simply cloning the sequence, assigning the contact owner to the prospects, and then enrolling the prospects into the clone to have a clean slate! Otherwise, instead of sending from Contact Owner, you may select one or multiple connected inboxes you'd like the sequence to send from using our Sending Distribution feature.
You can update the Contact Owner on the Contacts List page by running a filter for 'Contact Owner' 'is blank', and doing a bulk update.
Also, in place of the bulk update, you could actually have the contact owner assigned as the first step in the sequence automatically, if you are wishing to send from the contact owner. Please check out this article for more information on using this feature.
If you have confirmed that Contact Owners are indeed assigned to your prospects, and you are still seeing the message that Contact Owner is not assigned, it could be the case that whatever User is the Contact Owner does not have an inbox connected under their account. As an Admin, you could check this on your WarmUp page, and use the Filter at the top to filter by User.
For guidance connecting an inbox (email account) for sending, please check out the article here.
Once you connect an inbox to the specifics Contact Owner's Uptics account email, once the sequence enters the active sending times it should begin sending within an hour or so.