Student with Boolean Union Mesh errors

Hi! My name is Terrie and I am a university student working on a 3D project.
I transformed my models into mesh and I could not union them due to some kind of errors which I could find out why.
Is there anyone who can help me to see what’s wrong with my model?Thanks!

model.3dm (1.2 MB)

This is the model before changing to mesh.
model2.3dm (610.3 KB)

Hi Terrie - why did you want to convert to a mesh first and then stick it together?
Is this what you’re after:

model2_PG.3dm (433.4 KB)