this error occurs when spice can't run a DC analysis on your circuit, meaning that all DC node voltages are zero in your circuit.
it does not matter what type of analysis you are doing (AC,DC,TRAN,..) . SPICE does a DC bias analysis before ANY type of analysis and if it fails to find the DC values of all nodes, it reports floating point nodes.
solution : place a DC current source (IDC) and a very large resistor (say 100MEG) in parallel with your AC source (say your IAC or VAC). this will set your DC bias voltages, but has almost no effect on your AC analysis.