Why isn't my LED blinking?
EDIT: I checked https://www.robot-electronics.co.uk/files/iceWerx.pdf, I believe A5 and M4 are NOT the names of FPGA's pins - look for the pin names on the banks - those look to be P208 and P63.
If the input clock is indeed 12MHz, then counter[3] would be 1/2^3=1/8 of that (1.5MHz) - how do you expect to see that with a naked eye?
counter[24], OTOH, should be 1/2^24 of 12MHz, which is roughly 0.7Hz - you definitely should see that!
Some tools are very aggressive with optimization - check documentation if it's possible to inspect intermediate artifacts, like RTL netlists.
Are there any demo designs for the board? Do they work on your board? If not, then it's broken. If yes, use them as a starting point, cutting off pieces iteratively and checking if the remaining functionality still works.
As other suggested, you can also start with an even simpler design - just write constant 1 to one LED and constant 0 to the other.