Failed to validate boot image

I've got a Nexus 6 I gave a chick friend a year or two ago she brought back today that will not boot, it has a "failed to validate boot image: ret=-1" error in the bootloader log.
I can't get it to anything beyond the screen with Start in the center and a list at the bottom
of
AP Fastboot Flash Mode (secure)
BL: moto-spq8084-72.04(*) (sha-b11eacf, 2017-07-13 16:59:20
Baseband:
Product/Variant: shamu XT1103 32gb P3A0
Serial Number: (omitted)
CPU etc
eMMC etc
dram etc
Console [NULL]: null
Battery OK
Device is UNLOCKED. Status Code: 3

Connect USB Data Cable

I'm not an android guru, but I did have this unlocked and rooted and a 3rd party ROM on it last I saw it, but I don't remember what. Trying to flash recovery or a new rom don't seem to work and I'm beyond my skill level. Advice?