If the transition of interest is so close to the edge of a sideband that the half-width of the desired spectral window puts the edge of the window beyond the band edge, you need to either (1) choose a narrower spectral window, or (2) change the central frequency of the spectral window so that the spectral window falls entirely within the observable bandwidth.
To change the location of the spectral window, edit the "Center Freq" field in the spectral line table on the "Spectral Setup" page of the OT until the error message goes away, i.e. change it until the width of the blue line in the OT spectral visualizer falls within the yellow region that designates the extent of the sideband. It doesn't matter if the central frequency does not correspond exactly to the line transition, as long as the spectral range of interest falls entirely within the spectral window.
Observing lines close to the band edge and observing multiple lines is further described in the ALMA Technical Handbook.