Here is my demonstration of something similar to your assembly using the master modeling technique (Inventor 2021 format). Note that all the geometry is defined in the Master Part file, and that's where you would make any changes to the assembly. For example, try moving the mounting holes on the vertical square tube by adjusting the dimensions; then update the assembly, and see what happens. The parts making up the assembly are each derived from the master part, in which each part is defined as a separate solid body; expand the Solid Bodies folder to see each one listed, with all its defining features.
Now, some serious advice: you must get some training!! Your parts and assembly are a disaster. Some of your parts have no sketch dimensions at all, none of them are completely defined. You must learn, now, before you develop bad habits, to define everything, leave no degrees of freedom until you have a good reason to. I found that most parts were exact nominal dimensions, meaning that you used dimensions then deleted them (I can tell, because when I add a dimension it is, for example d11, not d0). Do not do that. Each sketch should say "fully constrained" in the bottom right corner before you are finished with it, and show a pin in the browser icon. As you look through the master part I created you will see that each sketch is completely constrained and dimensioned, and is a different color than yours-- sketch elements turn a different color, usually darker, when they are fully constrained-- actual color depends on the color scheme you have chosen. You want all of your sketch elements to be that darker color before you're done. We can deal with assembly constraints later...
Feel free to ask questions as you browse through this model. @JDMather is a master teacher, so he will have good advice and explanations, too.
One more thing, you need to update your Inventor 2021 installation. You are still using the initial release version, but 2021.2 is the current update level.
Sam B
Inventor Pro 2021.2 | Windows 10 Home 2004
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