Hello
Before setting up custom meeting backgrounds for your Teams Meetings, check to make sure you have the following items:
- Access to the Teams Premium SKU
- You’re an admin with access to the Teams admin center or you’ve been assigned a customization policy
- You’ve enabled the custom background policy
- Your background images meet the required specifications
If you navigate to Teams Admin Center < Meetings < Customization policies - you can add a theme to customize the look of your organization's Teams meetings with images and colors that represent your organization.
I would say key element here would be to check if Teams Premium SKU is added, if yes make sure to follow the steps mentioned here in order to have custom background within Microsoft Teams.
Please keep me posted if it answers your question.