The guide to install LineageOS tells you to wipe partitions in TWRP (“Installing LineageOS from recovery” steps 5 and 6).
That actually helps when something is wrong with an OS installation. Did you do something like that before earlier install tries?
Perhaps you did a factory reset before?
Mind, a factory reset on the Fairphone 2 doesn’t wipe partitions, it only deletes every user data and Apps and leaves OS and recovery as is, so if somehow the OS is corrupt, it stays that way after a factory reset. If somehow the system partition is corrupt, I guess it stays that way after a factory reset.
If you still feel like it, try to install Fairphone OS or Fairphone Open OS now. But wipe the partitions before that as you did with LineageOS.
You can easily install Fairphone Open OS with the TWRP recovery …
You can even use TWRP to install Fairphone OS (but in this case the manual install via fastboot is recommended) …