When adding an expense, you need to specify what the expense was for.

By creating an expense account or tax rate, when adding an expense, it gives you a list to select from.

To add an expense account or tax rate, click on '+ Add Expense Category/Tax Rate', or to edit an existing Expense Account/Tax Rate click on the pencil icon to the left of the named category in your list.

Enter a name for the account/tax rate, any additional notes, a colour and wether it is the default and click submit to add it to your list.