Why I am not able to Change Username and Tagline In Valorant?
You might not be able to Change Username and Tagline in Valorant because you might Change Username and Tagline a few days ago or you just created the new id in Riot.
Riot allows the player to Change Username and Tagline in Valorant once in thirty days. So, now you have to wait and come after thirty days to Change Username and Tagline in Valorant.