Some tournaments check every player on your roster before your team can enter. If your team is listed under Unable to join, the join panel names exactly what is missing and who it is missing for.
There are four checks. A tournament can use any combination of them, and the join panel only ever shows the ones that tournament actually asks for.
Game account
A linked in-game identity for the tournament's game — a Riot ID for League of Legends, a PUBG player name, and so on. League of Legends also needs a region.
Who can fix it: the captain or the player. A captain can add it from the team page using Add Game Account in the actions menu next to the player. The player can add it themselves under My Account → Game Accounts.
The name is verified against the game's own API when it is saved, so it has to match the account exactly. For PUBG, also check the platform: the lookup is per-platform, and a Steam name will not be found on the Xbox or PlayStation shard.
Game role
The position a player fills on the roster, for games that define roles — lanes in League of Legends, or Core and Sub in PUBG.
Who can fix it: the captain only. Use Assign Game Role in the actions menu next to the player on the team page. There is nothing for the player to do.
Steam account
A Steam ID on the player's own profile, used for server access and identity checks.
Who can fix it: the player only. It is on their My Account → Integrations page. A captain cannot link someone else's Steam account.
Discord account
A connected Discord account, so the player can be reached for scheduling and match coordination.
Who can fix it: the player only, from My Account → Integrations.
If you are the captain and the fix is not yours to make
For Steam, Discord, and game accounts, the join panel has a Remind button next to the requirement. It sends each affected player a notification telling them what to link and why, with a direct link to the right page. Each player can be reminded once per day per requirement, so pressing it again will tell you they have already been reminded rather than sending twice.
After everyone links
The checks run live. As soon as the last player links what they were missing, reopen the tournament's join panel and the team moves to Eligible teams. There is nothing to re-submit and no waiting period.