arachne-pnr: route.cc:652: void Router::route(): Assertion `cnet_net[cn] == nullptr || cnet_net[cn] == n' failed.

@RGD2 gawk '/^\.pins ct256/,/^$/ { print; }' /usr/local/share/icebox/chipdb-8k.txt (or simply open /usr/local/share/icebox/chipdb-8k.txt in a text editor and look for the right .pins section.

The lines in those .pins sections look like this:

B10 24 33 0
B11 23 33 1
B12 24 33 1

The first line means pin B10 is in the IO block with coordinates 24 33 and is in position 0 within that block. Looks like pin B12 occupies position 1 in that same IO block.