Hello !
Thank you for posting on Microsoft Learn !
What I have see so far, this is a common issue in Azure where orphaned serviceAssociationLinkremain on a subnet after a Microsoft.App/environments (Azure Container Apps Environment) is deleted, especially when set up using scripts like the one from Azure AI Foundry. These links can block subnet reuse and can't be deleted due to "allowDelete": false.
The serviceAssociationLink was created by the managed environment, but the automatic cleanup failed during deletion. Azure has no current public method (via CLI/Portal) to force deletion such orphaned links if the parent resource no longer exists.
The only way to resolve the issue currently is to open a support ticket since the link has allowDelete: false, somanual removal by Microsoft backend is required.
Meanwhile as a workaround, while you get the support, ceate a new subnet in the same VNet and update your deployments to use the new subnet as this avoids the blocked subnet but is not ideal if address space is constrained.