Every refusal names the rule and what to do next. Here is what each one means.
The site will not refuse an entry without telling you which rule stopped it.
Read the second sentence of the message. It says what to do next.
The reasons below are the ones people meet most often.
Age groups are decided by the year of birth, not by the age today. A player may always enter an older age group, never a younger one. For a veterans event the rule runs the other way: choose a band with a lower number, because 35+ accepts everyone that 45+ accepts, and more.
The message names the date and time entries closed. A late entry is the organiser's decision. Their contact details are on the tournament page.
The tournament page shows a Register button as soon as they do.
Open My Panel to see it. To move to a different event, withdraw this entry first.
The tournament limits how many events one player may enter. Withdraw an existing entry to change events.
Pair with a player of the other gender, or enter the men's or women's doubles event instead.
Team events are entered by the organiser, not online. Ask the organiser to add your team.