How to assign LVDS emulated pins on Cyclone V

I am trying to assign LVDS emulated pins to outputs of my top level design.
I am using an ALTLVDS_TX component to be able to send my outputs in LVDS format.
In the Assignment Editor, I assign pins like this:
LVDS_LCD_CAM_3_P Location PIN_E11
LVDS_LCD_CAM_2_P Location PIN_W21
LVDS_LCD_CAM_1_P Location PIN_Y24
LVDS_LCD_CAM_0_P Location PIN_AA20
LVDS_LCD_CAM_CLK_P Location PIN_D12
LVDS_LCD_CAM_CLK_P I/O Standard LVDS_E_1R
LVDS_LCD_CAM_3_P I/O Standard LVDS_E_1R
LVDS_LCD_CAM_2_P I/O Standard LVDS_E_1R
LVDS_LCD_CAM_1_P I/O Standard LVDS_E_1R
LVDS_LCD_CAM_0_P I/O Standard LVDS_E_1R

I checked in the Pin Planner that the negative pin is associated to the respective positive one.
When I compile the design, the Fitter stops and writing this message:
"Error (14566): Could not place 3 periphery component(s) due to conflicts with existing constraints (3 pin(s))"
"Error (175020): Illegal constraint of pin to the region (89, 18) to (89, 25): no valid locations in region"





What do I have to configure to get the fitter to place the outputs on the LVDS pins?

Regards,

Philippe