Hi @Rich7 
- Your screenshot seems to indicate that this member is a Limited Access Members. If you’d like this person to be able to remove/add members of your workspace, you’ll need to make them full member. More info in this article from our guide.
- First of all, please note that the Admin feature is only available with paid plans
When the current Admin is still active, you can easily re-assign admin ownership in-product (see the steps here), however in your case, since the Admin has left your company, you will need help from our support team. here are the steps to reach out to them.
Hope this helps, @Rich7 but let me know if you have any follow-up questions!