Interesting thing happened.
I’ve installed the most recent edition of TrueOS; and while attempting to dual boot with Windows 10 (something I ultimately kind of gave up on—I never use it anyway), I bumped into the same issue as the OP. It’s a Dell Inspiron 15 5000 series, stock save for the SSD manually swapped out for a larger HDD. After the first apparently successful installation, I noticed that my firmware had a completely empty list of boot options.
The second time through, I installed over the whole disk, to defaults, and still couldn’t start. After that, I switched to Legacy BIOS and suddenly it worked—am I correct in presuming that TrueOS is not entirely UEFI ready? I didn’t have Secure Boot on or anything, but it would not boot in strict UEFI mode. I kind of figured it was the opposite with BSD? If this is the case, it’s almost certainly impossible to install alongside Win10; which is a problem for most newcomers.
I’m still bumping into issues with sound, but who knows what that’s all about.
The other odd thing is that the thumb drive I installed from (built with disk dump, as suggested in the installation manual) would not boot in UEFI mode. That could easily be because of something I did, but if it is then it’s well hidden; I’ve been using *nix almost exclusively for about nine years.
Hopefully it applies in some manner to the OP’s issue.