First of all, DDR3 and DDR4 have fixed pin assignment options.

Documentation > Data Sheet tab > Packaging/PPATS
twist
Please download the Package Pin Assignment Table according to the part number being used.
https://www.microchip.com/en-us/products/fpgas-and-plds/fpgas/polarfire-fpgas/polarfire-mid-range-fpgas#Documentation

Because you can't assign any signal to any pin,
Please refer to the Package Pin Assignment Table to determine where to assign it (e.g. NORTH_NE, NORTH_NW, etc.).

* DQ can be bit swapped on the board. Please also refer to the FAQ below.
　 Microchip FPGA: Is it necessary to support Libero SoC when performing DQ bit swapping? - Semiconductor Business -Macnica

Open the I/O Editor in Libero SoC.
When assigning pins for DDR3 or DDR4, open the Memory View.

Select the DDR in the Main Object Browser and drag and drop it to the desired location on the right (NORTH_NE, NORTH_NW, etc.) and the DDR will be assigned to the pins all at once.

Note: Whether pin assignment is possible can be confirmed by checking the red or green color when dragging and dropping on the GUI.
Therefore, we do not recommend that customers write pdc and assign DDR pins.

After pin assignment:

Supplementary Note 1
If the following message about Data rate appears, try slowing down the Memory Clock Frequency (Mhz) setting in the DDR controller and running logic synthesis.
Try dragging and dropping again.
An example of a message
"Memory Interface 'PF_DDR3_C0_0(width=64, rate=1066)' cannot be placed at location 'WEST_NW'. Data Rate must be less than 800 for speed grade STD."

Note 2
We recommend that you assign DDR to pins before other signals.

Example:
I assigned the REF_CLK pins first.
After that, DDR cannot be placed in the intended location. (It can only be assigned to SOUTH_SW, etc.)

Solution:
Please try removing the REF_CLK pin assignment and then reassigning it to DDR.
Then, find the pin selection in the IO Editor pull-down that you can assign REF_CLK to.
If REF_CLK is grayed out and cannot be selected by pulling down the menu, the I/O standard of the pin to which REF_CLK can be assigned and the IO voltage of the bank may not match.
Try changing the I/O standard to see if that clears the greyout.