When you define varchar etc without a length, the default is 1.
When n is not specified in a data definition or variable declaration statement, the default length is 1. When n is not specified with the CAST function, the default length is 30.
So, if you expect 400 bytes in the @trackingItems1 column from stock, use nvarchar(400).
Otherwise, you are trying to fit >1 character into nvarchar(1) = fail
As a comment, this is bad use of table value function too because it is “multi statement”. It can be written like this and it will run better
ALTER FUNCTION [dbo].[testing1](@price int)
RETURNS
AS
SELECT ta.item, ta.warehouse, ta.price
FROM stock ta
WHERE ta.price >= @price;
Of course, you could just use a normal SELECT statement..
The maximal length of the target column is shorter than the value you try to insert.
Rightclick the table in SQL manager and go to ‘Design’ to visualize your table structure and column definitions.
Edit:
Try to set a length on your nvarchar inserts thats the same or shorter than whats defined in your table.