I can't say what the specific problem is without knowing more, but that particular entry is paired with Antling Sentries located just East of Camp Blackbrush (Central Than). You will have to kill 3 of the Antlings and 2 Tortoises to complete the entry.

The tortoise should be colored in however if you've killed 2. If not, you're either not killing the lv.12 ones in Hammerlea (West Than), or you have an actual bug.