The Uncategorized category is not a default site setting now since when General category is enabled on Discourse. On the new Discourse installs there is a default General category. So when you open the composer the automatically selected category is General. However on older installs where you haven’t general category, I think any category can be the general category. I think it is just means in this case the composer default category. So that won’t be enough if you create a category with General name. You have to set up the category id with the hidden site setting general_category_id too.
For example:
If you create a new category for this General purpose. The category name can be for example: General and you should check the category id as well. Now in this example it will be 9.
The category created successfully but if you open the composer it won’t be the default category so let’s set up the hidden site setting.
Set up category_id to general category
Note: category_id=9 is just in this example the correct category id. Your newly created category probably have a different category_id number.
cd /var/discourse
./launcher enter app
rails c
SiteSetting.general_category_id=9
exit
exit
After this, when you open the composer the category_id=9… General category will be the default.