Vlidating barcode scanner data entry in a text box, by If function

Hi all,
I have a userform, which is updated by a barcode reader.
When i want to validate one of these textboxes which has to have a fixed input, i can not use "else".
the code is as below,

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i want to show a msgbox after else, something like,

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but, when i do so, it'll only show the Msgbox by any changes.

Any Idea where is the problem, or what is the solution to do something like this?

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