Hi @Ec3-40 ec33469v ,
Could you try using the browser Teams to upload this file? Also in private mode open the browser.
After researching, this may related with the SharePoint site permission, please also see this thread: https://answers.microsoft.com/en-us/education_ms/forum/all/cant-upload-files-to-assignment/0aad6f7e-ea38-4856-af96-eb770fd8933b
Best regards,
Lou
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