I just checked the Anacapri website. There's a paragraph which mentions the problem you are having:
"It might occur that some of the symbols/keys (required to unlock doors/puzzles during the game) cannot be selected or are just not available. This is NOT a glitch, but a protection that takes place if some of the information or objects required to unlock a door/puzzle have not been obtained yet. If all the information or objects have been correctly obtained, relevant symbols/key will appear as they should."
I'd suggest to double-check that:
- You have pages 25-34 of the diary, taken from Massimo.
- You have pages 15-24 of the diary, taken from the Grotta under the Neolithic drawing.
- Read all pages of the diary in inventory.
- If you looked through the telescope and saw the gecko peeping out of the cave, move forward and examine the front of the cave.
Crossing fingers for you.