One of the categories is always set as the default category, which will be used for an article, if no other category is selected when the post is created. In your case it looks like Gallon tanks is the default one.
You can change the default category from Dashboard > Settings > Writing > Default Post Category. After you’ve changed the default category, you should be able to delete the unneeded category.
https://wordpress.org/support/article/settings-writing-screen/#writing-settings