Sandbox Refresh

In order for it to work correctly, at the time you are creating or refreshing your partial copy sandbox from Production, you need to specify in your sandbox template which objects it should copy data from and that must include the CreativeMail__Creative_Mail_Template_c object. This ensures that all the data from Production in that object is copied over to the sandbox and will ensure that the app works correctly.

Here is a link with more detail - https://help.salesforce.com/s/articleView?id=sf.data_sandbox_templates.htm&type=5