First, make certain that you don't have any volume interferences.
Second, make certain that all mating surfaces meet perfectly.
Third, start with a 4mm element size (minimum geometry dimension). That's your best 1st chance to get a halfway decent mesh.
You'll need to split the surface of those two cross tubes two ways to get a surface to apply your load.
James's advice is reasonable, and I see that it does lead to a successful mesh. With a mesh that fine, however, any studies will take quite a while to run.
You might consider rebuilding the frame as a weldment and using beam elements. This shouldn't take too long, as you have all the path information already. Your studies will mesh and solve in seconds.
We are looking for Designers like yourself who are working on ideas for the disabled.
If you have a moment check out....
Eventually it will fill in with SolidWorks folks that can help with all the "disciplines"