I get the vague error "Operation failed due to geometric condition". I have done this operation many times in the past with no problem. Would be nice if the error message were a bit more descriptive.
Can you please upload the file so we can take a closer look? Typically, this error occurs when some faces or edges overlap between the bodies selected for the combine operation, or if you have coincident edges.
Sometimes, creating a small overlap, will fix the error. The hard part is finding it. If you have parts that are touching along just one line, such as in the image below (blue line). SW can't join them. Touching at a tangent point (think of a cylinder resting on a plate) is another similar place SW will struggle.
This might be something where move face and moving one face on one part so you have a slight interference will solve the problem.
Vote for this: The "What's Wrong" dialog box Should have Links to Each Specific Problem & Better Descriptions
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