New Teams - unable to open a new Excel file created in Teams. Invalid file format

Hello Rob Butterworth 

welcome to the Microsoft community, my name is Recep I'll be happy to help you today.

I understand that you are unable to open an excel file created in New Teams and this is happening to some specific users.

please try the below solutions to repair your file.

Solution 1: Change the default file format.

Try changing the Excel default file format by following these steps:

In Excel 2010 and newer versions, click on the File menu.
Then, in the Info screen that appears, click on Options.
In the 'Excel Options' window, under the Save workbooks section, click on the Save files in this file format: drop-down. Next, select a file format (i.e., the one supported by your Excel version).
Click OK.
Check if you can open your Excel file. If not, try the next solution.

Solution 2: Edit the excel file permissions.

Follow these steps to change the file permissions and see if it fixes the problem:

Right-click on the file that will not open and click Properties.
In the file properties dialog box, click the Security tab and hit the Edit button.
When the file's permissions dialog box appears, click on the Add button.
Next, click on the advanced button from the 'Select Users or Groups' window.
Select Find Now. A list of all users and groups will be displayed in a search results box. Choose everyone's group from the list and hit OK.
You can see the 'Everyone' group in the 'Enter the object names to select' textbox. Click OK once again to return to the file's permission dialog box.
Click everyone from the Group or usernames: box and check all the checkboxes under Allow.
Click on Apply and then OK.