Cannot limit managed dropdown to a single selection.

I am using a managed column across several boards to manage meeting rooms. I want to limit the dropdown to a single selection, but when I try to select the setting, I get this error.

I checked the settings for the managed column, but I couldn’t find any reason that I shouldn’t be able select the setting. What is preventing me from doing this?

Note: I selected the option on a non-managed column, and it worked just fine.