I entered my payment method and I still can’t get my iPhone to backup.

Your question is extremely confusing. Here are a few reasons.

• Are you asking about:
  • iOS backup?
  • iOS update?
  • Payment method acceptance?
• What symptoms have you seen exactly?
• What are you trying to do?
• How are you trying to do it?
• What iPhone model?
• What iOS version?
• Is Wi-Fi involved?
• Is a computer involved? If so, using what software (name, version)?

I hope these links can help.

Back up iPhone - Apple Support

Update your iPhone, iPad, or iPod touch - Apple Support