Error(18948): Error message received from device: Internal error. (Subcode 0x001A, Info 0x00000000,

Ran into this issue also and noticed the workaround. If you are removing this assignment and have unused channels, this can cause degradation of those channels for future use.

Most likely the reason this setting is causing a programming error is that some of the unused channels are not powered up.

The unpowered channel is safe, but other unused channels that are powered up still need to be preserved to prevent degradation. This can be done using the following setting:

set_instance_assignment -name PRESERVE_UNUSED_XCVR_CHANNEL ON -to <pinname>

• <pinname> identifies the corresponding unused F-Tile for preservation.
• Preserves the entire F-Tile with a single pin. Pin can be specified on any lane, even if you do not connect the pin on the board.
• You can also use this .qsf assignment multiple times with corresponding pins from each F-Tile to preserve multiple unused F-Tiles.

Recommend taking a look at the Preserving Unused PMA Lanes section of the F-Tile Architecture and PMA and FEC Direct PHY IP User Guide.