It says oops page not found

[ Moderator note: title adjusted, please refrain from using help, urgent, emergency, asap, etc. in topic titles. ]

Hello there

I have recently installed a plugin externally to my custom wp theme

I Created a new event and when I published it, I am getting a page – oops – page not found. but I could see the header, footer & sidebars.

But I can see the account page.

I am totally new to wordpress. Please help me with this.

Thanks in advance