We understand your concern, Anonymous, and we're here to help guide you through this situation in a way that glorifies God. Firstly, let's remember what the Apostle Paul says in Philippians 4:6, "In nothing be anxious, but in everything, by prayer and petition with thanksgiving, let your requests be made known to God." (WEB)
It's commendable that you're seeking a role in a Christian charity. Leading in prayer is a wonderful responsibility, and it seems that God is calling you to step out of your comfort zone and grow in your faith. Remember, God equips the called, and He will not give you a task without also providing the strength and wisdom to accomplish it. As it is written in 1 Thessalonians 5:24, "Faithful is he who calls you, who also will do it." (WEB)
We advise you not to cancel the interview. Instead, be honest about your experiences. You can share that while you haven't led prayer in a professional setting before, you're eager to grow in this area and are confident that, with God's help, you can fulfill this duty. Your willingness and openness could be the very qualities they're looking for.
Here's a prayer for you:
"Dear Heavenly Father, we come before You in the name of Jesus Christ to lift up our brother/sister who is seeking Your will in their career. Lord, give them the courage to step out in faith and the wisdom to speak honestly in their interview. Equip them with the words to share and the heart to lead others in prayer. Help them to trust in Your strength and not their own. In Jesus' name, we pray. Amen."
Lastly, we encourage you to spend time in prayer leading up to your interview. Practice leading prayers, perhaps starting with close friends or family, and ask the Holy Spirit to guide you. God will honor your willingness and obedience. As Jesus says in Matthew 18:20, "For where two or three are gathered together in my name, there I am in the middle of them." (WEB)
We're praying for you, Anonymous. Trust in God, and He will direct your paths.