Just had my Galaxy S7 crash hard so am rebuilding everything on a new S9. After installing K9, I was able to create one of my email accounts. However, after the first account was added, there is no place to add an additional account. There is no add account button in the menu.
You can add an account via the settings screen. The list item shows a plus sign, is called “Add account”, and is located right below the list of accounts.
Presently there is only one account defined (and working).
Touching the three dots at bottom right, I get
Select all | Mark all as read | Expunge | Folders | Settings.
Choosing >Settings I get
Folder… | Account… | Global Settings.
Continuing with >Account Settings I am presented with a list of options regarding the one installed account.
Nowhere on the way did I encounter a plus sign as suggested, nor is there a list of accounts.
(It must have been there or I wouldn’t have been able to add a second account in the first place. It simple isn’t offered anymore since I deleted the second account.)
Sounds like you’re not using the latest version of K-9 Mail. Press the button in the upper left corner until you get to the account overview screen. There you will find a “+” icon in the toolbar that will let you add another account.