There can be multiple reasons for that.
1 – The sim is full. You will have to try again later as a spot becomes available when people leave.
2 – The sim is down for a restart. Try again later.
3 – You may have been banned. If you are a message will state so. This is not something we can get involved in. If you want to try to resolve this please contact the land/buoy owner. Do NOT discuss them in groupchat!
4 – It’s a skilled game region. To enter skilled game regions, there are certain rules. Those can be found here.