The system consists of two inverters ( Quattro and Fronius), batteries and PV panel. previously, the system was connected to a 50 Hz grid. Now I would like to connect it to a 60 Hz grid. Should I connect the inverters to the 60 Hz grid first and then change the frequency settings using VRE Config.? .I need the sequence to change the frequency of the inverter( 50Hz to 60Hz).
I have changed the frequency. However, there is no output power from Quattro. The mains on flashes and the float mode flashes as you can see in the picture. Fronius inverter is working and supplies power to the grid but it's not appearing in the picture due to the wifi was disconnected. 
I cant see the flashing lights as its a photo, and on looking up lights error sequence that you say is occurring that BOTH the mains on and the float is flashing. There is NO error code for that.
you are on a 230v 60hz power supply and not a 120v 60hz (just double checking)
if you turn the mains off does the inverter start inverting and supplying 230v at 60 hz to the loads
double check that you did send the changes to the quattro , so plug in the mk2/mk3 and recheck the changes
You should be using the below to configure two units that if you have two quattros that is
I have one Fronius inverter and one Quattro inverter. The frequency of both inverters is set 60 Hz. In addition, to better describe what is happening, the LEDs of the "mains on" and the "float" are on as you can see in the photo (not flashing). but the problem there is no output power from the PV chargers (MPPTs). the power available comes only from the Fronius inverter.
thats because the batteries are full thus no more charging is required and the Quattro has not been setup to export power to the grid
If you want to export power to the grid then you would setup say a ESS assistant
