As you know, users cannot be completely deleted from your Bitrix24 account. They can only be dismissed. This is done in purpose - to keep all your data safe. When the workgroup/project creator is dismissed, the workgroup/project is not affected. Further, you may need to either appoint a new owner for this group, move the group to archive or delete it completely.
First of all, click your profile picture at the top right corner > My Profile.
Activate the Admin mode by clicking the small arrow next to the Administrator sign > go to the Workgroups section.
How to change the workgroup owner if he/she has been dismissed
Select a workgroup with a dismissed owner > click Actions > Edit Workgroup to change the workgroup owner.
Click Change owner.
Select a new workgroup owner > click Save.
You have successfully assigned a new workgroup owner instead of the dismissed one.
How to delete the workgroup if its owner has been dismissed
Click the Actions button > Delete Workgroup. Remember that this action will delete the workgroup completely and irreversibly.