Erik Kostson
Ansys Employee

First since this is a truss make sure you use only one element per member (so you should have 3 elements in total - make the mesh size large enough, say 5000 mm or 5 m that you get only one per member). Otherwise it will not work if you are using truss elements (with beams it will work).

For fixed support use it on the start and end vertices of your model not on the whole truss - so start vertex and end vertex of your model. That will then be as in your exercise where it is fixed at both ends.

Finally and most importantly, make sure that you have connected parts in your cad via multi-body part in design modeler or sharing topology in space claim - if you do not do that they will not be connect and it will not solve.
You can also still connect the members at their common vertex via merge nodes in mechanical (see help manual and search for merge nodes - if you do this you will get probably that it merged 2 nodes), that will share the nodes so they have common nodes there and are thus connected.

Finally I recommend watching some videos online on how to model with beams and trusses.
If you search (beam and truss and ansys workbench) you will get many hits on search engine.

All the best