Hi kelvin_wijaya_1992 •,
Welcome to Microsoft Q&A forum and thanks for using Azure Services.
As I understand, you want to create Azure Cosmos DB for Mongo DB API using vCore cluster in Japan East Region.
For Azure Cosmos DB for MongoDB cluster, currently we support only 3 regions and in future other regions will also appear as the service evolves.
We are sorry for the inconvenience you are facing due to this temporary limitation.
I recommend you to try for the above-mentioned regions or would request you raise support case so that team could exclusively enable your account for the desired region.
If you do not have support plan, please let us know so that we can help with other options to unblock you.