Hi Bridget,
Thanks for using Microsoft products and posting in the community, I'm glad to offer help.
According to your description, I created a list with the same Choice column as yours, but I cannot reproduce the issue. There is no error if I set a default value for the choice column.
In this case, please go back to the modern experience and edit the Choice column, checking if you can set the default value from the edit pane.
Besides, if you don't mind, you can create a new Choice column or a new list to see if the problem persists.
Looking forward to your feedback.
Best Regards,
Arck