The "Title" column comes default on any list you are going to create in SharePoint, it's also by default a "Single line of text" type. This introduces problems if a) You don't want a Title column or b) you want that Title column to be a lookup or another column type.

If you have searched on how to remove the Title column from a SharePoint list you probably found a few articles on how to edit the "Item" content type to hide the Title column. This solves your issue if you just want to solve a) above. But you'll notice if you try to create a new column named "Title" with a different column type, say Lookup, you'll get an error that Title still exists.

I will quickly run through resolving both a) and b) if you still want "Title" but want it to be a different column type.

1. Go to Settings > List Settings > Advanced Settings on the list in question
2. Click YES to allow management of content types and click OK
3. You will now see the "Item" content type under "Content Types", click "Item"
4. Click on the Title column
5. Select the radio button to make "Title" Hidden and click OK.
6. (Optional if you want to make a new "Title" column under a different column type) Click Title again and then click "Edit column in new window". In that new window change "Title" to anything else, make sure "Require that this column..." is set to NO. And click OK. Close the new window and click OK on your main window.
7. You will now see Title is no longer there when you click "New"
8. You will want to go through any views you have and remove any references to the know hidden column. You can also now create a new column, name it "Title" and give it any column type you want.