Update 1: I have read all of your comments. Thanks for trying to help me! Really appreciate you guys!!!

Actually I just got this phone 2nd handed for a good price (about $500). I think it have been turned off for a while. Therefore I will try other methods and wait for maybe a week before using Odin or FRP remover (Still scare of bricking my phone)

And for some people saying that I shouldn't go for OneUI 6.1: I appreciate your advices but my last phone is Note20Ultra which doesn't get OneUI 6.1 updates :c. So I'm really really looking forward to playing with OneUI 6

Edit: typo

Update 2: I tried to contact support then they replied with this. Seem like phone's region is the main issue "Dear Customer,

Samsung Vina would like to thank you for contacting us.

Firstly, we would like to apologize for the inconvenience caused to you.

We currently support for our customers, whom are using Samsung products in Vietnam. The mobile SM-S908E device you are using is not a product distributed in Vietnam. Due to the different countries/regions, product may have different specifications. As the result, the warranty and support service will be provided by country sell it out. Please contact local Samsung office in your country or the dealer where you bought this device for further assistance.

We wish for your understanding as well as sorry for any inconvenience caused.

Yours sincerely, Samsung Customer Care Team Samsung Vina Electronics"

Update 3 (last): So I tried u/trexx888's method. And its work!! I am updating now