TWRP fails to flash stock ROM (invalid zip) I have a sprint samsung galaxy s5 (g900P)
I am a mac user without access to a PC (no odin). 
I have TWRP recovery installed. 
I am currently running lineage OS (nougat)
I need to return to stock (the reason for this is that I am trying to activate new cellular service and it doesn't seem to work under lineage. If someone has experience activating TPO or other cell service using lineage or updating PRL on non-stock firmware please let me know since that might obviate these other problems).

I used android file transfer to move the stock firmware zip in the download section to the internal SD (root folder). I wiped the system and cache, then install zip - but it fails every time with "invalid zip file format". I have tried a half dozen other versions of the stock ROM from various sources and they all fail with the same error. ZIP signature verification is disabled. Just when I thought that i was doing something completely dumb and not flashing correctly at all - I tried re-flashing the lineageOS ROM i have and that worked fine. I've read a bit about unpacking and re-zipping these zips to get them to work, but that requires 7zip (which I don't have on mac). Help! am I doing something dumb or do I not have the correct zip files?

I saw this thread that had a partial answer but I am not a developer so it was way over my head. not sure I know how to proceed: (SOLVED - partial) Flashing Source Built ROM Fails in TWRP, Invalid File Format
Do i need to modify the zip files I have? can someone link me to a zip for the stock firmware that will work on TWRP? 
for the record I grabbed the 3CQA2 zip from here: https://www.mediafire.com/folder/brujjvaro2ogo/Odin_images
which I grabbed from this thread https://forum.xda-developers.com/sprint-galaxy-s5/development/firmware-sprint-s5-factory-odin-images-t3585129

Thanks!
I encountered this too:

build/tools/releasetools/sign_target_files_apks -o -d "$KEY_DIR" out/target/product/$DEVICE/obj/PACKAGING/target_files_intermediates/aosp_$DEVICE-target_files-eng.xx.zip $OUT/$TARGET_FILES

build/tools/releasetools/ota_from_target_files --block -k "$KEY_DIR/releasekey" $OUT/$TARGET_FILES  $OUT/$DEVICE-ota_update-$BUILD.zip

build/tools/releasetools/img_from_target_files -v $OUT/$TARGET_FILES $OUT/$DEVICE-img-$BUILD.zip


TWRP will install $DEVICE-ota_update-$BUILD.zip, but failed to install $DEVICE-img-$BUILD.zip: Failed to install, invalid zip file format!. fastboot update $OUT/$DEVICE-img-$BUILD.zip works.

TWRP version twrp-3.1.1-0-grouper