You might be facing the above issue if you purchased using a server other than Malaysia or your MLBB account is from a foreign country other than Malaysia. Please note that you can no longer purchase using mobile payment methods such as Maxis, Celcom, Digi & U mobile.
You can only make purchases with other payment methods such as e-wallets. [E.g: ShopeePay,
Touch n Go, etc.]
This is a new setting by MLBB. In addition, you should also know that after a successful purchase, you will get a different amount of diamonds if your account is from a foreign country.
For more information, please refer to this link for the changes - https://my.support.codashop.com/hc/en-us/articles/4406402903695-Changes-in-Mobile-Legends-Bang-Bang
We hope this helps.