The column is too narrow to display the value (or a negative date/time).
Fix this with AI##### isn't really an error — it means the cell value is wider than the column, so Excel can't show it. It also appears when a date or time formula produces a negative result.
Widen the column
Double-click the column border to auto-fit, or drag it wider.
Fix negative dates
Ensure the later date is first in a subtraction, or format the result as a number.
Not usually — it's a display issue. Widen the column and the value appears.