What does it mean that "Annexation is estimated to be Unknown"?

Annexation time is unknown since you have negative rate of progress -- see your second screenshot. You have -3 diplomatic reputation, which is due to annexation of Choctaw within the last 10 years. A Statesman advisor will allow you to integrate Cherokee in the current circumstances, if one is available and you can afford him.

From EU4wiki page:

Annexing occupies a diplomat until complete. The base cost of annexing a vassal is 10 diplo points per base tax multiplied by hostile core creation modifiers, spent at a rate determined by

• +1 base
• +1 if same culture as suzerain
• +1 if same religion as suzerain
• +or - suzerain's diplomatic reputation (rounded down).

If the total is 0 or negative, annexing will pause, but not go backwards. When all the diplo points are spent, annexation finishes and the suzerain will see the following effects:

• -3 Diplomatic reputation for ten years. Doesn't stack. [...]