Additional features, settings, or issues not covered by specific Microsoft Teams categories
Hello Gina
Thank you for participating in the Microsoft Community, I am Skypper G. I am an independent advisor, let me help you with this.
This is a default behavior since excel in teams is Excel Online and that feature does not coming with it.
In order to use the Fill Function you will need to open the excel file with the Excel Desktop app.
In the link below you will find a couple of workarounds that may achieve your request: