Since there are a lot of people complaining about why they can't sign in for the OBT event, I hope that this will help you guys out. The previous days I was able to claim my OBT Taruh Semua but I also experienced the problem of not being able to claim it.
1st day I was able to claim the 2 Taruh Semua both the 20:00 and 21:00. During the 2nd day, I logged on at around 19:55 and I wasn't able to claim Taruh Semua 20:00 gift box but at 21:00 I was able to. The next day, At 19:30 GMT-6 I was playing my two characters at the same time. My main character was doing slots (refreshing) and the other one was just staying at the village. At 19:59 my main entered the village and checked on the website for the time, refreshed at 20:00 but I wasn't able to sign in while my other character refreshed at around 20:02-20:03 was able to sign in. So what I did was 30 mins earlier before the last claiming which is 21:00. I let my two characters just stayed at the village and patiently waited for 21:00. At 19:59:xx I opened the OBT Taruh Semua event window and clicked the sign-in even though I can't click it (lol) and refreshed at 21:00. Surprisingly, the two of my characters are able to sign in
So I came up with this theory that you should:
1) Log in for about 15-30 mins earlier before the time.
2) You can do stuffs like quests etc. while waiting but DO NOT refresh once you are logged.
3) Once you are signed in, AVOID refreshing or closing your window, stay online for 30 mins to claim the reward ( It happened once to me. I was able to sign in but not able to claim the reward )
I do not guarantee that this will solve your problem but hopefully it works for you too cause it worked for me!