The comment field is not shown on a card.

There are two possibilities why there is not a comment field:

1. The card does not accept comments. Such cards include quiz cards and test cards.
2. The provider of the learning material has disabled this function.

If the card is neither a quiz card nor a test card, and you still cannot see a comment field after logging out and logging in again, it is likely that the reason is 2 of the above.