Recently I wanted to update the Firepower Module in an ASA5516-X from version 7.0.1 to 7.0.4. The readiness check failed with the error message "Platform architecture x86_64 is not supported. Supported architecture(s) are i386. ". According to the TAC engineer, the following workaround on the sensor worked for me:
1. remove the upgrade lock:
rm -r /tmp/upgrade.lock
2. rename the /ngfw to /ngfw.old
mv /ngfw /ngfw.old
After that the readiness check could be done successfully. During the upgrade, however, the error message "no upgrade status package found" was displayed. To solve this problem I had to do the above steps again and after that the upgrade worked as desired.
According to Cisco the bug is CSCvy95809.