Hi,
Does your company apply any sort of mobile app policies that might be only allowing file interactions with say OneDrive for Business?
That might be worth a try as a workaround too. So upload file from device to OneDrive presuming that is an option and then try and add/share via OneDrive.
Although having said the above if you are uploading a file from device in a Teams chat, it essentially uploads to OneDrive and then shares.
So to that end, do you have OneDrive for business signed into on your device with the same account you are logged into Teams with.
If you don't have OneDrive for business that too could be the issue.
Or are you trying to upload as part of a Teams channel post or just files tab of a Teams channel as that would involve SharePoint Online.
Thanks
Henry