Which TWRP version are you using?
TWRP 3.1.1-0 from Fairphone would be known to give this error, although this TWRP version shouldn’t be on the phone anymore (Fairphone dropped it and instead bundled the stock Android recovery with the last Android 7 Versions of their OSes).
Else the error could simply be caused by “Zip signature verification” being enabled in TWRP’s Settings (you can get there from the main menu).
Is that even TWRP? Sounds more like the stock Android recovery, which would be in line with what Fairphone bundle with the OS version you are running.
In this case TWRP is available at twrp.me including how to install, or for a quick booting of TWRP without installing it there’s #twrpwoflashing.