The Notes about mass and gravity loads are reported every time when reference loads are used. This message says that if you would use an automatic seismic load generator, the mass model would be formed using the reference loads of 'Dead' and 'Live' load type as reference load with 'Mass' or 'Gravity' load type is not present in your model. As long as you do not use an automatic seismic load generation, this message does not have any meaning for you.