They are very open about it on the world status page, which does not requiring logging in https://eu.finalfantasyxiv.com/lodestone/worldstatus/

It specifies:
* New characters cannot be created on Congested Worlds.
*The creation of new characters may also be restricted on other Worlds, depending on the level of congestion.

I studied this myself prior to buying and subscribing the game, where with the free trial I was unable to create a character on my desired world and studied how to get around it. Back then, we didn't even have world visit or cross-world party finder. Now we do, so you can communicate across worlds using a Cross-World Linkshell, you can visit their world and go to them at their FC house.

If you already upgraded from the free trial, the launcher usually contains a link to the latest announcement on which worlds have Congested status. It's not actually their fault if you don't study the information they present in front of you. If the world does not have Congested status, then you actually can make a character, it just depends on the time of day and how congested it is.