Hi Loos, Michael,
It is a pleasure to be able to help you :)
Here are two options available for this issue:
Option 1: You can log in to this site using the previous owner account if you have and add a new user to the default owner group, then the user will be granted full control permission as the owner.
You can do this by following the steps below:
1.Click the Settings in the Upper right corner, select the Site permissions.
2.Select the Advanced permissions settings
3.Click the Owners Group, and click New to add users to this group.
4.Enter the user's email address and click Share. The user then receives an invitation email. You can go to the site as instructed by the email.
Option 2: You can log in to SharePoint admin center with a URL https://tenant-admin.sharepoint.com/, add new users in the site owner group.
Note that this requires a tenant administrator account. You can refer to the following steps:
1.Go the admin center with URL https://tenant-admin.sharepoint.com/ like below, sign in with your tenant administrator account.
2.Click the Active sites in the left navigation, and search the sites you want to set and click it.
3.Click the Membership and select site owners, click Add site owners, enter the user's email address, and Add it.
4.The user then receives an invitation email. You can go to the site as instructed by the email.
Hope this helps.
Best Regards.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.