KKBOX is currently available in Taiwan, Hong Kong, Macau, Malaysia, Singapore and Japan.
Ensure the country/region of your Google Play is set to the country your KKBOX account is registered under by referring to Google Help.
You should find KKBOX under Google Play after that.

NOTE:
Devices purchased in Mainland China may be equipped with Google Play that does not support In-App Purchase, therefore you will not find, or is restricted from installing KKBOX.