@CB1929 I remember having the same issue and I think I solved it by first putting the phone in fastboot mode, connect phone to computer, start the Ubports installer and then select by hand in the Ubports installer the Xperia X Performance phone.
I also needed to do this: Flash the OEM binaries from: https://developer.sony.com/file/download/software-binaries-for-aosp-nougat-android-7-1-kernel-4-4-tone/ using fastboot flash oem SW_binaries_for_Xperia_AOSP_N_MR1_5.7_r1_v08_tone.img , because even with the updated Android 8 it did not boot.

Hope this helps.

Most of the things work great! I also managed to install anbox. That went well for the biggest part, however there is no network or wifi available within anbox. How can I make that work? Perhaps some kernel modules are missing?