SQL Server datatypes nvarchar and varchar are incompatible error

You should use the + for string concatenation:

SELECT 
  ResolverID AS ddlValue, 
  ResolverTeam + ' | ' + ResolverPerson AS ddlText 
FROM dbo.TblResolvers 
Order By ResolverTeam, ResolverPerson;

Why am I getting this error?

You were getting that error, because of the & operator, which is the Bitwise AND.

Is this a concatenation attempt? ResolverTeam & ' | ' & ResolverPerson

& is the bitwise operator AND, replace it with + and see what happens.

Try replacing the & for a +; by the looks of it, what you're trying to do is to concatenate 2 columns. Something you do need to be careful about is that nvarchar is double the size of regular varchar, which means there are chars in nvarchar that are not in the varchar table.

To concatenate strings in MSSQL you should use +

SELECT ResolverID AS ddlValue, 
  ResolverTeam + ' | ' + ResolverPerson AS ddlText 
    FROM dbo.TblResolvers Order By ResolverTeam, ResolverPerson;