I'm not sure about some of these nice and informational players' Fleet's who have been so kind to help you and invite you(kudos BTW), but many with T3+ Starbases want new recruits to contribute 200,000 Fleet Credits into Fleet Projects before allowing you access to purchase items from their Fleet Store. This is to prevent ppl from jumping Fleets and clearing out supplies, then just leaving and doing it again elsewhere. There's not an infinite number of Fleet 'weapons' or 'shields' available, they have to be 'provisioned' (or resupplied) by the Fleet Projects.
Don't be discouraged by this, it's not an impossible goal. But like I said, not all Fleets work this way mostly the bigger ones. Good luck to you.