Ephesians 6:18 - "praying always with all prayer and supplication in the Spirit, being watchful to this end with all perseverance and supplication for all the saints—" (emphasis mine)
We can see this in the physical world around us. When a young person goes into the military, they just don't hand them a weapon and place them on the front line. They send them through Boot Camp. They receive the training that they need to be able to do what they are sent to do. When a person goes into the Medical field, they are trained and go through an internship to be sure they are prepared for treating others. When a person goes into the field of education, they take courses in their field, they student-teach and thus become qualified to teach others.
So, God also qualifies those whom He calls. So, what does Paul mean by "in the Spirit?" Well, first a person must have a salvation experience with Christ (Eph. 1:13-14). One must have heard the truth, trusted in Christ, and thus are sealed by the Holy Spirit. Then, we must not harbor sin, or have any unconfessed sin (Eph. 4:25-30). John Bunyan is credited with saying -"Prayer will make a man cease from sin, but sin will make a man cease from prayer."
In Ephesians 4, there is a list of sins that we are to remove, these grieve the Holy Spirit. Included in the list is lying, anger, thievery, worthless words (gossip, criticism, mean spirited talk) as well as others. We can't be in sin and in the Spirit at the same time. We can't pray and hate at the same time. We mus be yielded to God's Spirit (Eph. 5:15-16). And in turn, we will be filled with the Holy Spirit (Eph. 5:18). When we are this focused in our spiritual walk, then we will be ready to be a Prayer Warrior.
Prayer: Spirit of the Living God, as I meditate upon the truth from this passage, may You drive it deep into my being. Light the fire in my soul to be involved in the Kingdom Building Business of being a Prayer Warrior. Amen.