If you intend to make the app available to play store users then you need to create a play store account. Normally the warning that you received if from Android OS telling the user that the app can't not be trusted, since it does not come from the play store but yourself built it. You can ignore this "warning" for now.
The warning will not be shown to the users when the app is built, signed and uploaded to play store.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…