Jeremiah Cornelius

Also, I realized you probably can't just sideload the /e/ image! You can follow the LineageOS instructions for F(x)tec and get a clean install from your self-built ROM: https://wiki.lineageos.org/devices/pro1/install Best, — Jeremiah

I hope these links are some help. The answer is that you need a new TWRP, for community and experimental builds. The official build TWRP has signature checking, regardless of the unlocked bootloader. Related to your issue, here on these forums is advice for the F(x)tec TWRP and experimental builds. More recently, from the /e/ forums, there's this solution described by a user and the OnePlust 7TPro. The solution is similar to that described above, with his method of updating TWRP - tho' I caution you NOT to use his link for the OnePlus TWRP! 😉 https://community.e.foundation/