You are not allowed to change your home currency after the first transaction is recorded. You should open a new QBO Global version account for Cambodia (i.e Essentials or Plus), enable the multi currency feature, and set USD as the home currency before recording any transactions.
https://quickbooks.grsm.io/Asia-Pacific
You can utilize a migration tool to migrate data from your old account prior to canceling it.
https://transactionpro.grsm.io/qbo
Otherwise, consider purchasing a 3rd party migration service to help.
