There are 2 modes by which design can be done in Staad, the batch mode and the RC Designer mode. The batch mode is the normal mode where the analysis and design is done. The RC Designer mode is specially for concrete design purpose and for BS 8110 it is recommended to use the RC Design module. Kindly see the attached screenshot.
Also, I have used BS8110 for the batch mode , and the corresponding model is also attached.
A hogging moment of nearly 420 KNm is developed at the start of the beam , the depth provided 300mm is not sufficient for it . As you increase the depth ( or width , but preferably the depth ), the design is carried without any such warning messages.
As you are new to Staad, if you have any problems with the analysis and design of your models, kindly create a Service Ticket and Bentley Technical Support will help you.