I've this constraint inside a table:

CREATE TABLE [dbo].[InventoryLocations]
(
    [recid] [int] IDENTITY(1,1) NOT NULL,
    [LocItemNumber] [char](16) NOT NULL,
    [WareHouse] [char](2) NOT NULL,
    [Aisle] [char](3) NOT NULL,
    [Slot] [char](3) NOT NULL,
    [locLevel] [char](2) NOT NULL,
    [Bin] [char](2) NOT NULL,
    [Extra] [char](2) NOT NULL,
    [LocNumber] [char](2) NOT NULL,
    [RollNumber] [char](20) NOT NULL,
    [QuickRoll] [int] NOT NULL,
    [SkidNumber] [char](15) NOT NULL,
    [RollsInStock] [int] NOT NULL,
    [LocQtyOnHand] [float] NOT NULL,
    [LocQtyOnOrder] [float] NOT NULL,
    [LocQtyCommited] [float] NOT NULL,
    [TotalReceived] [float] NOT NULL,
    [TotalIssued] [float] NOT NULL,
    [TotalDollars] [float] NOT NULL,
    [Capacity] [float] NOT NULL,
    [AvailableSpace] [float] NOT NULL,
    [bkey0] [char](30) NULL,
    [bkey1] [char](30) NULL,
    [bkey2] [char](30) NULL,
    [bkey3] [char](14) NULL,
    [LastPhysicalCountDate] [datetime] NULL,
    [LastCycleCountDate] [datetime] NULL,
    [EnteredBy] [varchar](50) NULL,
    [EnteredDateTime] [datetime] NULL,
CONSTRAINT [IX_InventoryLocations_1] UNIQUE NONCLUSTERED 
(
    [LocItemNumber] ASC,
    [WareHouse] ASC,
    [Aisle] ASC,
    [Slot] ASC,
    [locLevel] ASC,
    [Bin] ASC,
    [Extra] ASC,
    [RollNumber] ASC,
    [SkidNumber] ASC
));

So when attempting to place the next rows, I recieve the mistake message. In the cursor which populates the variables.

INSERT INTO [AVANTISERVER\NCL_MASTER].[Avanti].[dbo].[InventoryLocations](LocItemNumber, WareHouse, Aisle, Slot, locLevel, Bin, Extra, RollNumber, LocQtyOnHand, SkidNumber)
            SELECT @item, 'F', 'L', 'E', 'X', 'O', @seq, @seq, @qty, @seq
            FROM FI_CurrentReceiptData CR

Which calculates to:

VALUES('MW1', 'F', 'L', 'E', 'X', 'O', 0, 0, 10, 0)
VALUES('MW1', 'F', 'L', 'E', 'X', 'O', 1, 1, 10, 1)

Monosodium glutamate 2627, Level 14, Condition 2, Line 34 Breach of UNIQUE KEY constraint 'IX_InventoryLocations_1'. Cannot place duplicate type in object 'InventoryLocations'.

Help me to know why I am unable to make these insertions? I'm not familiar with these kind of complex constraints. Exactly what does that one mean (basically) and just how can one circumvent it? Shall We Be Held really breaking the constraint?

do you have rows within the table that conflict using the data out of your question that you're attempting to place?

EDIT after comments and question edit
add this:

put some prints inside your loop to determine what values you are placing. I do not think SQL Server will problem a mistake unless of course you are attempting to place dups.

All the products inside your SELECT list are constant expressions.

INSERT INTO [AVANTISERVER\NCL_MASTER].[Avanti].[dbo].[InventoryLocations](LocItemNumber, WareHouse, Aisle, Slot, locLevel, Bin, Extra, RollNumber, LocQtyOnHand, SkidNumber)
            SELECT @item, 'F', 'L', 'E', 'X', 'O', @seq, @seq, @qty, @seq
            FROM FI_CurrentReceiptData CR

I am unsure the reason why you expect it to differ between rows but the point is it will not. Possibly you designed to reference a column as opposed to a variable somewhere.

While you say within the comments these variables are now being up-to-date inside a cursor most probably your SELECT is coming back multiple rows FROM FI_CurrentReceiptData. Comment the Place and check out the outcomes and find out the number of rows are came back. Or simply remove FROM FI_CurrentReceiptData CR entirely because the SELECT does not use everything from it.

Not understanding what they are called from the posts you are attempting to INSERT, simply by the number of posts indexed by your constraint (9) it appears like each row should be completely unique within the table InventoryLocations. You are placing (10) posts. Your constraint is determined wrong in line with the data you need to add.

EDIT: After your table definition edits, you've still got a lot of posts indexed by the UNIQUE CONSTRAINT that is leading to the INSERT failure. It might help much should you listed the column names you are placing to.

EDIT: You're placing hard-coded values into posts indexed by your constraint. Given that they will be exactly the same the constraint is violated and also you begin to see the error you published. You have to either vary the information starting the table or relax the constraint by getting rid of a few of the posts.