Please have a check on your site storage here. You shall be able to see the used storage of the site. You can have a try to managed the storage limit of site with issue like this doc said: https://learn.microsoft.com/en-us/sharepoint/manage-site-collection-storage-limits
If that does not help, you may need to open a Microsoft support ticket. And you can post in this forum. There are experts who may provide useful advice.