I'm doing the mission “Elder Knowledge” to get the elder scroll, but since I’ve already asked him about an elder scroll because of the Dawnguard DLC, he doesn’t have the option to talk to him about it, but the mission won’t continue. I’ve tried closing the game (on PS4) but he still doesn’t have the option. (In Skyrim: Special Edition)
From the Wiki, this appears to be a known bug:
If all of the quests are completed for the College of Winterhold before having talked with Urag gro-Shub about the Elder Scrolls, there will be no text option to ask for the books about the location of an Elder Scroll. The proper text option has a slim possibility to appear after all available chat options have been selected. This bug appears if the Dragonborn has already spoken to Septimus Signus before talking to Urag. If Septimus Signus has already been talked to, it is possible to skip talking to Urag and continue on to the Tower of Mzark
Usually, as is the case suggested in the excerpt, you can simply move on to the next part of the mission, and force the progression of the questline.