We can't clearly see or identify an insect in the photo, and we can't see all of the damaged berry, but there are several possible culprits for fruit damage: slugs/snails, birds, and sap beetles being among the most common. Sap beetles tend to feed on overripe fruit, so are less likely here since the berries aren't ripe yet. Have you inspected the fruits/plant at night? Many plant-feeding insects hide during the day and feed at night, so a flashlight inspection after dark might reveal the culprit. Of course, if birds are pecking at them (either to eat an insect atop the fruit, or to nibble on the fruit itself), they will be harder to discourage short of covering the plant with netting or a cage, or trying a scare tactic like putting out a fake owl, fake snake, or strips of shiny Mylar to blow in the breeze to startle them.

Our Growing Strawberries in a Home Garden web page provides more general tips for keeping the plants vigorous and productive. If you catch an insect or other animal in the act of chewing into the fruit, we can then help ID it and determine how to protect the berries.

Miri