If your phone is a mainland China version device (such as OPPO, Huawei, Xiaomi, etc.),
you may encounter a situation where clicking to buy doesn't bring up the Google Play Store purchase message.
Please go to "Settings," select "App Management," then "Google Play Store," and find "Background pop-up display" in the relevant settings area.
Set it to "Allow."
Then return to the VoiceTube app, and you should be able to make the purchase normally.