Valorant, Riot's competitive tactical shooter, pits teams of five players against each other as they attempt to attack or defend bomb sites. The lightly hero-based shooter has a heavy emphasis on weapons, and as you'd suspect from a free-to-play game, it has a huge selection of skins you can drop real money on.
If you've tried to buy something from the store, like Valorant Points which allow you to buy in-game skins, but you're still getting a "purchase failed error", then unfortunately you can't. not do much to solve the problem since your end. Valorant will sometimes run into issues with the store's servers and simply won't properly process your purchase requests.
For Valorant, restarting the client might be a magical fix for most issues, but it might not fix this one for you, and any store instability will need to be fixed by Riot. If the problem has persisted for some time, it is advisable to submit a support ticket with Valorant Support.
The good news is that a failed purchase won't cause your Valorant Points, the game's premium currency, to disappear, so there should be no worries.
Issues like this don't normally stick around for long, as Riot can be pretty quick to fix issues that affect the store, so hopefully you won't have to deal with them for too long.
