Bixby voice isn't available for the A20. Just Bixby home which you already have. Bixby voice is embedded in the firmware and isn't downloaded.
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Have you tried going into your amazon account and removing the tv from devices? You can then try logging in the tv as a new device and see if that helps.
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You can try enabling it, change your settings, and then disable it again through 'permissions' in settings. Not sure that will work but it's worth a try.
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