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From the uesp article about that quest:

To continue past this point, you must be level 15 or higher. If you aren't level 15 yet, giving Septimus the Lexicon will result in him dismissing you; then, you must wait until a courier delivers a letter from him to continue.

I don't know why the letter hasn't triggered, but you could use the console command setstage DA04 40 to force yourself to the next stage of the quest.

Alternatively you could try leaving and returning to towns to trigger a courier.

From the UESP article about that quest:

To continue past this point, you must be level 15 or higher. If you aren't level 15 yet, giving Septimus the Lexicon will result in him dismissing you; then, you must wait until a courier delivers a letter from him to continue.

I don't know why the letter hasn't triggered, but you could use the console command setstage DA04 40 to force yourself to the next stage of the quest.

Alternatively you could try leaving and returning to towns to trigger a courier.

From the uesp article about that quest:

"To continue past this point, you must be level 15 or higher. If you aren't level 15 yet, giving Septimus the Lexicon will result in him dismissing you; then, you must wait until a courier delivers a letter from him to continue."

I don't know why the letter hasn't triggered, but you could use the console command

 setstage DA04 40

  to force yourself to the next stage of the quest.

Alternatively you could try leaving and returning to towns to trigger a courier.

From the uesp article about that quest:

To continue past this point, you must be level 15 or higher. If you aren't level 15 yet, giving Septimus the Lexicon will result in him dismissing you; then, you must wait until a courier delivers a letter from him to continue.

I don't know why the letter hasn't triggered, but you could use the console command  setstage DA04 40 to force yourself to the next stage of the quest.

Alternatively you could try leaving and returning to towns to trigger a courier.

Frome the uesp article you linked "To continue past this point, you must be level 15 or higher. If you aren't level 15 yet, giving Septimus the Lexicon will result in him dismissing you; then, you must wait until a courier delivers a letter from him to continue."

I don't know why the letter hasn't triggered, but you could use the console command

setstage DA04 40

to force yourself to the next stage of the quest.

Alternatively you could try leaving and returning to towns to trigger a courier.

From the uesp article about that quest:

"To continue past this point, you must be level 15 or higher. If you aren't level 15 yet, giving Septimus the Lexicon will result in him dismissing you; then, you must wait until a courier delivers a letter from him to continue."

I don't know why the letter hasn't triggered, but you could use the console command

setstage DA04 40

to force yourself to the next stage of the quest.

Alternatively you could try leaving and returning to towns to trigger a courier.