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Each of your cities has more than three neighbors. Assuming none of those are drop package assets you are trying to get rid of, sell all but three that touch each one (the bottom core has 9 neighbors, for example).
For the three you keep, the SURGE asset for those three must be different (food, bucks, energy, for example).
Then, of the three remaining, upgrade them to your current level.
Last, check the core - you should NOW (and only now) have an UPGRADE button, providing you have enough bucks to actually pay for the upgrade. If you don't, then wait a turn to see if more bucks will fix the problem.
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