I can't buy a ticket
Disabled buttons, sold-out tiers, closed sales and events not taking payments.
The reason is usually written on the page. Here is what each one means.
"Sold out"
That tier has no capacity left. Check whether the event has other tiers — they sell out independently. Otherwise the organizer would have to add capacity.
"Sales closed"
The event's registration deadline has passed. This is the organizer's setting, so ask them if you think it closed early.
"This event isn't accepting payments yet"
The organizer has not finished connecting their payment account. Nothing is wrong on your side. Save the event and check back.
"The organizer hasn't published a ticket link for this event yet"
This is an externally ticketed event with no link filled in. There is genuinely nowhere to send you. Contact the organizer.
There are no tickets at all on the page
Either the event is free — in which case just turn up — or it is externally ticketed and you should be seeing a Buy ticket button that takes you off-site. If the event has already ended, the ticket section is replaced by a photo gallery.
The buy button sends me to sign-in
Buying requires an account. Sign in and you will be brought straight back to the same checkout.
Payment itself is failing
That is covered separately under When a payment does not go through.