Your code is plain wrong, but since it look like HomeWork, no full blowup solution.
Here is how you would organize your code
loop until user have entered a UID
prompt user for new ID
get user input
loop for every item in inventory
compare item ID with new ID
if it match, new ID is not unique, ask again
end of loop
end of loop
insert the new ID in inventory
When you don't understand what your code is doing or why it does what it does, the answer is
debugger.
Use the debugger to see what your code is doing. Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute, it is an incredible learning tool.
Debugger - Wikipedia, the free encyclopedia[
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Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[
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Basic Debugging with Visual Studio 2010 - YouTube[
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The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't find bugs, it just help you to. When the code don't do what is expected, you are close to a bug.