Note: my character, Ace Gokh, is a male Orc who began TESO in a cell in Coldharbour, from which The Prophet (an NPC) freed him. He is associated with the Daggerfall Covenant Alliance. Currently, The Prophet is in The Harborage in Glenumbra. Your questions sent in reply to my request for help are in bold font in this response:
What is the name of the quest you are experiencing issues with?
The name of the next quest in the Main Quest series is "The Tharn Speaks". However, currently, it is not recorded in my character's Quest Journal.
What quest objective are you experiencing this issue with?
My character (Ace Gokh) has not received the initiating instruction to find Abnur Tharn -- from whatever source it is given. That is why it is not recorded in his Quest Journal, of course.
What is the issue that you are experiencing with this quest?
Abnur Tharn is
not waiting for Ace Gokh in the Mages Guild Hall in Daggerfall (Glenumbra). There would be a large white downward-pointing arrowhead above the NPC if he were there.
To quote the wiki created by Enodoc, a TESO Community Ambassador,
Online: The Tharn Speaks :
"Upon completion of the last quest, you should get a quest telling you to go to the Mages Guild in your alliance's first major city (Vulkhel Guard, Daggerfall or Davon's Watch). Inside the guildhall, the quest will update; now you need to talk to Abnur Tharn. Tharn is dressed like a mage in blue and white robes. ...."
In sum, short, and fine: a character cannot begin a quest unless and until the quest is received (and accepted) from a "quest-giver", which is usually an NPC. (Although a quest can be given by an object such as a corpse, a document, a weapon, jewelry,
etc., which the character finds and "examines".)
Thank-you for your time and effort in providing a resolution of this strange situation. In the course of playing four other characters during the past five-plus years, this is the first time that I have found my character unable to continue the Main Quest story-line because the character did not, and could not, obtain the next quest in the series.