Good catch. --use-syn-sdc now has no effect when --ignore fieldmaps is used. This is a bug, and I will report it.
Do you have fieldmaps you’re ignoring? If not, then you should be able to drop --ignore fieldmaps and get what you want. If not, you’ll need to wait for a fix or revert to an older version. 1.5.2 is the most recent version that should not exhibit this bug.
Just to confirm that I am also receiving this error on some (not all) subjects. I thought this was related to improper setup of field map parameters causing fmriprep to try and fall back to syn-sdc, but maybe I was wrong. I am currently re-running a subject that had this problem with fmriprep 1.5.5, and if the problem is still there will try 1.5.7.
P.s. We don’t typically need syn-sdc, we just produce it for comparison with field map sdc.