PLEASE HELP! I can't get TWRP on oneplus3t

I have a oneplus 3t and I'm trying to get TWRP and eventually Nethunter on it (my computer is Mac). I unlocked the bootloader no problem using adb reboot bootloader and sudo fastboot oem unlock. I know it's unlocked because of what it says on the bootloader screen.

Now, I want to get TWRP so I tried:

fastboot flash recovery twrp.img (twrp.img came from their site, it's the one for oneplus 3t)
fastboot boot twrp.img

but the phone just hangs at the teamwin start screen (the one with the little blue arrow motif) and the only way I get rid of it is with adb reboot bootloader.

I have the general idea that this is from a problem with dm-verity, but I really have no idea what to do to correct it. If somebody could let me know, that would be fantastic. I really want nethunter and I can't make it work without TWRP.

Thanks guys

Well I sort of fixed that but now I have another problem. If I only run fastboot boot twrp.img then I get into twrp no problem. I started trying this again with the ./twrpFlash.sh script, but I consistently get errors like

Unable to locate zip file '/sdcard/supersu.zip'.
Installing zip file '/sdcard/supersu.zip'
Checking for MD5 file...
Skipping MD5 check: no MD5 file found
Failed to map file '/sdcard/supersu.zip'
Error installing zip file '/sdcard/supersu.zip'
Done processing script file

It doesn't matter whether I do the commands by hand or not or whether or not I use the -p option for push, it refuses to work. Any advice on this front?