Hello-
Are you using a purchased copy of v2018? The trial version is limited to burning 30 seconds of video.
If you purchased it, open the Nero ControlCenter and check to see if your serial number is shown correctly (39 characters, including hyphens, and only zeroes (there are no "O"s.)). If the trial version is shown there, remove it.
Although the video you're trying to burn is commercial and possibly copy protected, I don't think copy protection is an issue since it let you import it and it's allowing you to burn it to the image recorder.