The parameterized query ..... expects the parameter '@units', which was not supplied

Try this code:

SqlParameter unitsParam = command.Parameters.AddWithValue("@units", units);
if (units == null)
{
    unitsParam.Value = DBNull.Value;
}

And you must check all other parameters for null value. If it null you must pass DBNull.Value value.

Here's a way using the null-coalescing operator:

cmd.Parameters.AddWithValue("@units", units ?? (object)DBNull.Value);
cmd.Parameters.AddWithValue("@range", range ?? (object)DBNull.Value);
cmd.Parameters.AddWithValue("@scale", scale ?? (object)DBNull.Value);
cmd.Parameters.AddWithValue("@description", description ?? (object)DBNull.Value);

Or for more strict type checking:

cmd.Parameters.Add("@units", SqlDbType.Int).Value = units ?? (object)DBNull.Value;
cmd.Parameters.Add("@range", SqlDbType.Int).Value = range ?? (object)DBNull.Value;
cmd.Parameters.Add("@scale", SqlDbType.Int).Value = scale ?? (object)DBNull.Value;
cmd.Parameters.Add("@description", SqlDbType.VarChar).Value = description ?? (object)DBNull.Value;

The operator also be chained:

int?[] a = { null, null, 1 };
Console.WriteLine(a[0] ?? a[1] ?? a[2]);

This extension class was useful to me a couple of times so far, for those issues:

public static class DbValueExtensions
{
    // Used to convert values coming from the db
    public static T As<T>(this object source)
    {
        return source == null || source == DBNull.Value
            ? default(T)
            : (T)source;
    }

    // Used to convert values going to the db
    public static object AsDbValue(this object source)
    {
        return source ?? DBNull.Value;
    }
}

You would normally use it in two scenarios. First, when creating parameters for your query:

var parameters = new Dictionary<string, object>
{
    { "@username", username.AsDbValue() },
    { "@password", password.AsDbValue() },
    { "@birthDate", birthDate.AsDbValue() },
};

or when parsing the SqlReader values:

while (reader.Read())
{
    yield return new UserInfo(
        reader["username"].As<string>(),
        reader["birthDate"].As<DateTime>(),
        reader["graduationDate"].As<DateTime?>(),
        reader["nickname"].As<string>()
    );
}