First thing: don’t Panic! 
You did the Wipe before, right?
In TWRP, do the following: go to “install”, select “lineage_FP2-whatever.zip”, uncheck “Zip file signature verification”. Than “swipe to confirm flash”.
If you want to install open GAPSS as well, repeat the last step (of course without the wipe…). You can download them here. Choose ARM/7.1/nano.
Than reboot.
And remember: don’t panic!