Warning: This page allows direct access to your site settings. You can break things here. Please be cautious!

I am currently developing a wordpress theme and it has 2 custom admin settings sub pages which is created with wordpress settings API. However, in first sub page i can apply settings and save without any problem. But in second subpage, when i click 'save changes' button i get an error like this.

Warning: This page allows direct access to your site settings. You can break things here. Please be cautious!

Does anyone know sort it out? I have no much of experience in wordpress theme development and this is my first theme.