I want to cancel a pending purchase so I can pay for Apple Music

You cannot cancel a pending purchase. You must let it complete correctly and charge you. Once that happens you can request a refund for it. Link-> Request a refund for apps or content that you bought from Apple - Apple Support

Here is a one-stop link for billing questions→Subscriptions and Billing - Official Apple Support. You can review your purchase history, cancel a subscription and request refunds.

What type of pending purchase are you wanting to cancel?

Normally Apple grants refunds without any issue. You won’t be able to access the store until that is resolved though.