The reason it isn’t solid is that you haven’t bothered to make it a group or component. (Funny, that was my guess way back in my first post. All you need to do is select the geometry and make it a group or a component.
How do you find the problem? Well, I found it by clicking on a face with the Select tool and noticing only that face was highlighted. Had it been a group or component, there’d have been a large blue bounding box around it.
So I guess the how do you find it comes down to experience. Learn to use the program correctly and gain some experience as Box suggested…
In order for a component to be considered solid it must contain only faces and edges. No other components or groups inside the container. And all edges must be shared by exactly two faces. No more and no less. This means you can’t have any internal faces and it can’t have any holes. You can see internal faces if they exist using X-ray face style. And you can see holes if they exist by the presence of of heavy profile edges where they shouldn’t be and potentially by the appearance of reversed faces.
In addition, all faces need to be oriented correctly. All white faces out. The blue back faces must be on the inside. Face orientation isn’t an issue in this model but it’s not an uncommon problem for new users to have.
It isn’t clear from your model. Is the 3D text supposed to be part of the model or is it just there as a label?