Dear user,
Bc4 (algebraic notation)
or
f1c4 (coordinate notation - rarely used in human readable chess publications)
The same holds true for the other attempts to move the bishop 'bd3' and 'be2'.
Hence I think that GNU Chess was right when it complained. I just verified it on version 6.2.1, the latest stable version.
However, before version 6.2.0, the program was permissive with piece names except precisely for the bishop. This means, that lowercase letters were accepted for all pieces except for bishop, which had to be uppercase at any rate. I think that this may be what you are experiencing.
Our general recommendation is always to upgrade to the latest version (6.2.1), and use the algebraic notation as is. If you need to stick to your current version, you must use 'B' for bishop.
Thanks for your interest and do not hesitate to contact us again.
Regards,
--Antonio Ceballos