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

Question

I'm playing as the Creek nation in North America with the Cherokee as a vassal. My mission is to absorb the Cherokee into my country. A few years ago, I integrated the Choctaw without issue, but the dialogue box for annexing the Cherokee says:

Annexation is estimated to be done Unknown due to 3 basetax.

This message continues for a few months, so I stopped trying the diplomatic annexation. I sent another diplomat to Improve Relation with them, but even after boosting it to the limit, there's no estimate of when the annexation will be done. Here's the tooltip:

Is this a bug or is there something I can do to annex the Cherokee peacefully?

Answer 1

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. [...]