There's still many issues with this, but I think the kata could be OK, so I'll list the problems I found so far:

• Initial code should compile. Right now it's missing 'export' keyword before types, and you need to account for nullable type in output.
• The example in description is STILL incorrect. f3 and g2 should be 'b'. I would suggest instead to take the first sample test and put it in description, it's much easier to see what's going on then.
• expected { Object (before, after) } to deeply equal { Object (before, after) } is not acceptable form of feedback. The fix is very simple and shown below by FArekkusu.
• You should move board initialization into it blocks before calling assert, so input and expected output is close to each other.
• Suggestion: add a fixed test with 'w' in outer circle, and all others 'b', then change some middle element. This should be a good confirmation if solution is correct.
• You NEED random tests. Solve some other katas and click 'Show Kata Test Cases'. In short, you need to generate a random board, compute the expected result, and compare with user's solution.

Good luck!

No random tests.

assert.deepEqual(
  simpleReversi(board2, [4, 3]), {
    before: ['b', 36],
    after: ['w', 45]
  }
);

This test is wrong - there're 48 white pieces after the reversal.

The error messages are useless.