this is a "Lookup" column and you cannot change the type of a lookup column to another type. You will have to delete and re-create that column
You can only change types between the following column types
Single Line of Text
Multiple Lines of Text
These are the "simple" column types that internally more or less work the same....
The other types like "Lookup" and "Managed Metadata" Fields store data differently internally and have more possible settings. For example are two fields created if you create one "Managed Metadata" field - the managed metadata column and a hidden multiline text field. That is why you cannot easily map back from them to a "simple" column type.