import ucs without f5mku

not quite sure what you are asking and is going on.

f5mku is relevant when you have encrypted objects in your UCS, lots more details can be found here:

https://support.f5.com/csp/article/K9420

i assume you have and can't do the f5mku command? why not, better would be able to perform that command by getting the value from the original system.

you can change an UCS file, it is just a tar.gz file which you can extract, change and compress again.

before you load it the system should be on the same version, be licensed and perhaps some other things, consult the documentation again.

as for the error, show the /var/log/ltm to determine where it fails.

You can remove the passpharse from bigip.config file and load the configuration.

or

If you have another peer device you can run the same command and get the key