There are two most common reasons.

  1. You have missed the start of a seasonal race. If you either joined a co-op or checked off Take part in seasonal regatta after the regatta had already begun, you can follow your co-op's progress in the regatta, but you can't complete tasks.

Co-ops competing against each other are grouped together based on their size. It wouldn't be fair to give an advantage to any co-op by allowing them to add a new member to the race. There's a one-day break between regattas that gives you time to change co-ops.

KEEP IN MIND: if you leave your co-op before the end of a regatta, you won't be able to claim rewards from the race.

  1. A co-op leader or a co-leader has restricted you from participating in the current or upcoming regatta, depending on whether a regatta was in progress when the restriction was imposed or not. If that happens, you'll get a notification in the co-op chat. If you wish to take part in the next seasonal regatta, check off the corresponding point in Settings. You can still take part in the next interseasonal regatta regardless of your status.

You can find more information about restricting players from regattas in the following article How do I restrict a co-op member from participating in regattas?.