TWRP is fine.
I have lineage-15.1-20180415-UNOFFICIAL-cedric.zip in the Downloads folder on the 'phone.
On attempting to flash this, TWRP fails with: "Updater process ended with ERROR: 255".
Advice elsewhere suggested unzipping the package, then removing the first 2 lines of file at META_INF>Com>Google>Android>UpdaterScript. These refer to the 'Cedric' device name - but I have no real idea what they mean.....
I did this, but as I'm on a mac, could not follow the instructions relating to re-creating the archive as they were Windows specific ('Add to rar' was suggested). So I tried straight zip archiving.
Attempting to flash the archive thus created, TWRP now fails with : "Invalid zip file format!".
Would be grateful for advice on how to create the appropriate file format using a mac.
I have an app called 'Keka', which offers as options 7z, Zip, Tar, Gzip, Bzip2. Are any of these appropriate?