AS suggested, set the AMP ASSIGNMENT to the BASIC option in the configurations. Run the YPAO calibration and this will detect that there are no back surround speakers present. There is no requirement to use the BI-AMP assignment opyion in the absence of back surrounds, presence speakers or a second audio zone.
As to getting Atmos, the receiver will not acknowledge or portray Atmos in the absence of presence speakers. You’d have to have at least a 5.1.2 speaker setup to get Atmos and for the receiver to acknowledge the presence of Atmos metafata. If you’ve no height, ceiling ot up[ward firing presence speakers within your setup then the receiver will not indicate you are receiving or portray Atmos. Your AV receiver is not acknowledging Atmos because you’ve nort a speaker setup that can portray Atmos.