The command you need to use is
jav-it /c r18.com_cookies.txt /i rdb00978 /o <folder where you wnat your video>
I'm guessing you did not enter your cookies file name correctly

Yes but without the " and also remember to change the <> to folder you want your video to be in. Folder needs to exist first too.
And that /i is for the video ID, I only wrote you that rdb00978 for it ready because that was the video you showed you already have

You can use any folder you want, aslong it exists.

I wonder if the issue is with your cookie files name.. try renaming it to cookies.txt and using that?
If that does not work, there is some sort of issue with your cookies-file itself

No, it will be in TS-format so you will have to convert it to mp4 yourself. Simplest way would be using Handbrake because you don't have to mess around with command lines.

It happens to me too when the R18 video is split in 3 or 4 parts. The last one is corrupted most likely because the R18 session to access the movie is expired.

Delete the bad file. Get an update cookie file of the movie and re-run the same command. Jav-it will detect part 1, 2, 3... are already on disk until it reach the part that is not downloaded.