Hello Jessica,
Good day! Thank you for posting to Microsoft community.
From your description, I understand that you are having issues adding a co-organizer in your meeting.
Firstly, please note that external users (users how are not in the same organization as you), cannot be added as co-organizers, so it needs to be someone from your org.
To add a co-organizer, Open a meeting in your Teams calendar. > Make sure the people you want to add as co-organizers are added as required attendees > SelectMeeting options:

Select Roles
> and select users you want to make co-organizer > Apply:

Or please schedule a meeting as you normally do, and send the invite >after go back to your calendar and right click on the meeting > select edit > and click on meeting options:

It will open in the browser, so sign in with the account you used to create the meeting > Under meeting options go to roles > and select the meeting co-organizer > and then save.

For more: Add co-organizers to a meeting in Microsoft Teams - Microsoft Support [d331-9acf-7ec-3963]
If you face any error or get stuck in the process, please let me know by responding to this thread, I am more than happy to help.
I look forward to your response to this. Please note that our first response does not always resolve the issue immediately. Only with your help and more detailed information can we find a solution. Thank you for your help.
Thank you for your cooperation and understanding.