there is something wrong with my JuMP!
It used to work!why?
thank you!
Looks like some package version conflict with CapacityExpansion, JuMP, and/or BiLevelJuMP. At least when I install CapcityExpansion it downgrades the preinstalled JuMP to version 0.19.2.
You could try to remove these packages and re-install them. You may also try to add these packages in an empty project-environment, and see what happens.
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thanks！it works! 
Here’s the issue: https://github.com/YoungFaithful/CapacityExpansion.jl/issues/25.
@abcde If you need CapacityExpansion.jl with JuMP 0.21, it’s probably not too much work to help them update. Probably mostly just a few find-and-replaces.