Bootable SSD and converting to APFS

I boot my iMac from an external SSD, which is a clone of my main HDD, running Mojave. I had no issues before, recently with the Catalina being released I downloaded the update and when I choose my SSD drive I get: "this volume is not formatted as APFS".

What should I do now? I tried from recovery to convert it, but the option is disabled.

Is the only way to format it? or is there another way I am not aware of?

It turns out there was no way to change my SSD to APFS without formating. I ended up using "carbon copy cloner" cloned my SSD to internal hard drive, then formatted my SSD to APFS, and cloned everything back to SSD. All good now and I was able to upgrade without trouble.