Hello TO KA (M,
Good day and thank you for reaching out!
I understand that you are having issues editing and replying to an attached Excel file in the preview screen of Office 365 web version Outlook email. May I confirm if by preview you mean the very first option in the drop-down?

If so, I did some tests on my end and the preview opens only in read only mode with no edit permissions. Quick consults showed that some other users have made consultations on this functionality, and I'll have to check further with the team.
Also, as an organization admin, you can also raise a service request to report this symptom to our online channel through the O365 admin center if you have permissions. Our dedicated channel can further connect with you remotely, for further troubleshooting and involve the backend team where need be and verify whether this is due to a design change.
Get support for Microsoft 365 for business -- Online support.
I will also revert back when I get more info on this.
Your patience and cooperation are much appreciated.
Best Regards,
Ashraf.