The following 3 things need to be checked first:
1) The subscription plan is created in live mode (looks like you did this already)
2) The “Live Mode” option is set in the settings menu is checked. Please verify this.
3) The product doesn’t have the “Force Test Mode” option enabled (this option allows you to override the setting for individual product).
Next, delete the subscription plan and create a new one (so it gets created in live mode using live mode API keys. Use this new subscription plan in the product and check it. Does it still give the same error?