You won't be able to do the Menagerie more than once until you've completed The Invitation.
The last step before the upgrade should have requested you to claim your rewards from the Triumphs. You should have had at least 2 Triumphs, one for completing it and one for beating the boss. Both of those should have given you 1000, not 500.
Please check your Triumphs for the Menagerie and collect the rest of your rewards, then you'll be able to purchase your upgrade for 1000.
English
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Oh wow, I don't often look at my triumphs, I find it a bit dumb to put rewards in only a small part of them. Anyways I got way more imperials that I had to have thanks to you.