Hello Stuart MacMillan
Thank you for reaching out and informed us about the issue with Copilot app on your device. Since you're using a personal account but the error “Your organisation does not allow this action.” appears.
However, you also mentioned that if you download the file directly from your OneDrive to your device first, then you can upload to Copilot. From what I gathered, I believed that currently you're signing in to OneDrive using a work/school account. If that the case, according to Connecting Microsoft Copilot to other services the account type from Copilot and OneDrive must be matching in order for Copilot to be able to attach the file directly through Connector:
Therefore, I would advise checking to see if your OneDrive session is logging in to a work/school or a personal account, otherwise you will need to manually download the file to your device first before attach to Copilot.
I hope this helps.