Thank you for your reply.
I looked, but it did not help me much. Constraints and loads are listed in the Model Tree (model has no errors when checked), yet I get message saying that constraints are missing.
Analysis number was always listed on the bottom of the Tree on all my successful analysis, but this one does not show analysis number.
To be slightly more explicit: the reason you have three load and constraint sets listed for your analysis is that it's possible to use sets from parts or sub-assemblies in higher level assemblies. This is exactly what you've done: you've selected the constraint set from the part, which presumably is empty, not the constraint set from the assembly which you're looking at in the tree.
It would be helpful if Simulate would show the file extension (either .prt or .asm) when listing the sets in the analysis definition - but it doesn't.