Distribute Spacing field is disabled if you set a key object BEFORE opening Align panel
Windows 10 Pro (64-bit), Illustrator 24.0.1 (64-bit)
If I wish to align / distribute two or more objects in Illustrator, if I select the objects and click on the 'master object' BEFORE I go to the 'Align' tool window, the distance box remains greyed out as you can see in one of the screenshots below. I find this very annoying, for example, I may have previously been on the 'Transform' tool window sizing my objects, then I copy them a few times and select them all, select a master object then go to the 'Align' window and the distance box is greyed out. I then have to re-select everything and re-select the master object while the 'Align' tool window is open for the distance box to become active. I find this very annoying and Illustrator CS5 didn't used to behave in this way. If this could be looked at please, it would be brilliant, thank you.

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Jeremy commented
Distribute by centers is in the control bar when more than one object is selected, but distribute by spacing is missing.