How i can fix that please?
This is security/Permission issue. Here is the resolution for you:
- Navigate to SharePoint admin center.
- Click on BCS settings.
- Click on Manage BDC Models and External Content Types.
- Look out for your required BDC Model and then select Set Metadata store permissions to Execute for all the membership groups.
- Click on Secure Store Settings.
- Select the required Target Application ID.
- Click Edit and Add the required groups. For example (Everyone except external users) to Target members group.
This will resolve your permission issue and you can select your external fields Source.