Dear Carla1969,
Good day! Thank you for posting to Microsoft Community.
I have been testing your query from my side and based on my test result, I can able to set default scheduling policy time increment of 2 minute and the feature works as expected. You can refer to my test result screenshot:

However, if 2 minute increment is already set, it will show you disabled "Save" button when you enter 2 minute. Therefore, in order to understand your query and provide possible solution, May I please double confirm if you tried to create a service page and check if the time increment is already set 2 minute as default time increment? If you haven't check it yet, In your Bookings -->select Services -->Add new service -->Give name for your service -->Availability options, check if Time increments already set as 2 minute. You can refer to the screenshot below:

Alternatively, you may try to set the default scheduling policy 5 minute increment and change to 2 minute. To do this, kindly choose "5 minutes" on first and click Save. Then, again choose "2 minute" and check if the Booking page allows you to save.
I look forward to your update. Please feel free to let me know how it goes or if I got you wrong. If you still have any concerns, please let me know so I can provide more suggestions.
Thank you for your precious time. Have a nice day.
Sincerely,
Libeamlak | Microsoft Community Moderator