In what way did it not work?
- original?
- repair attempt?
- the implication is that the install method finished prematurely. This has been reported to happen silently (or un noticed).
The manual install script avoids the problem of the title by flashing both slots.
(eosinstaller is actually expected to do the same.)
Easiest thing may be manual install Install /e/OS on a Fairphone The Fairphone (Gen. 6) - “FP6” rather than debug the installer.
[HOWTO] a Safe Guide to install "IMG-e-builds" even if the /e/ web installer fails