Warning: Please check the form carefully for errors!

I have just installed Opencart Version 2.3.0.2 and I have been trying to safe my store settings ; but each time I tried to safe the store setting, I get the error message mentioned in the subject above :"Warning: Please check the form carefully for errors!". I will really appreciate if someone can assist in providing solution to the error message as it is impossible for me to move ahead with other works that I need to do on the site ; the website is plentyofmart.com.
For a clear idea of what I am talking about, I have attached a screenshot of the issue.

It is a form validation error and it means that you are trying to save form without filling all mandatory fields.

Please check "Store" and "Option" tab and ensure that all mandatory fields have some value. After that when you will save setting, this error will not come.