Hi Wajih Arfaoui,
Thanks for reaching out to Microsoft Q&A.
it plainly looks like a permission issue. Are you sure all the permissions are already there for synapse to access the external table file location?
Are you using SPN or MI to read the external table? In addition, to Storage Blob Data Contributor provide Storage Blob Data reader role as well.
What about the firewall? are there any blocks.
Please 'Upvote'(Thumbs-up) and 'Accept' as an answer if the reply was helpful. This will benefit other community members who face the same issue.