Work down this list in order. It is ordered by how often each one is the answer.
1. Are you on your club's address?
This is the answer most of the time.
Your club has its own web address — yourclub.swim.win, or the club's own
domain. A SwimWin session belongs to one club, and it is created on the
address you signed in from.
Sign in on the wrong address and you get a session your club's screens refuse.
Check the address bar. If you bookmarked swim.win rather than your club, that
is the problem.
2. Are you a member of that club?
If the club has not approved your registration yet, you cannot sign in to it. The message you get is the same as a wrong password — deliberately, so that the sign-in page cannot be used to find out who belongs to a club.
If you registered and heard nothing, chase the club, not SwimWin.
3. Did you ever set a password?
Registering through the join form does not create an account. The account comes with the invitation after the club approves you. If you never got an invitation, ask the club to re-send it.
4. Is it the right email?
Whichever address the club has for you. If a household has two parents, each has their own login — you cannot use your partner's.
5. You belong to two clubs
Then you have two sessions on two addresses, and signing in to one does not sign you in to the other. This is deliberate.
"Forbidden" after signing in successfully
A sign-in that works and then every screen fails is the club-address problem above, seen from the other side: the session is real, it just belongs to a different club than the screens you are looking at. Go to your club's address and sign in again.
A shared tablet signs in as the wrong person
Somebody closed the tab instead of signing out. Use Sign out in the account menu — closing a tab leaves the session behind and the next person picks it up.
Still stuck
Tell your club, and tell them the exact address you were on and the exact message you saw. That is the one detail that turns a twenty-minute conversation into a one-minute one.