Even though relevant command are active what is the reason dimensions are not showing in assembly level?
Do you have any features (cut-extrude, boss, hole, etc...) in your assembly (specifically, at the assembly level)? If you don't then no feature dimensions will show up.
If you go to a part with features, then select the same selection, you will notice the feature dimensions show up.
It only shows dimensions at the same level that you are at.
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