what screen you are using in ECC for 0021?
2079369 - IT0021: Option for Undeclared gender not available > should be implemeted in your system . in order to use the feature of undeclared gender and other US specific fields - screen number 2010 must be used. Also, your error seems to be some mandatory field is not filled too? can you check for USA in table V_T588MFPROPC, what fields are mandatory?
check steps here 2228959 - Unable to hide the 'Undeclared' gender field on Infotype 21