Unable to Select Shared Field for Multi-Select Text Input Field

I have a table that consists of different department tags, such as HR, Operations, Research, etc. I have a separate table that has projects that get tagged to multiple departments. I want to create a multi-select text field that uses the "Department" text field in the other table as a source field. When I try this the pop-up opens and I am able to select the correct field, but when I click "Ok" the field does not update.



The "Select shared field" option is now grayed out and when I try to save I get the following message:



Any and all suggestions welcome!