From Word's point of view, OnMerge Barcodes are inline images. This means that the barcodes are initially created at the current text cursor's location, and are positioned as if they were part of the surrounding text.

Unlike other inline images you may be used to working with, you cannot change OnMerge Barcodes formatting to make them positionable, e.g. by using Word's controls to Size and Position the barcode, or by changing its Wrap Text setting. Likewise, you cannot place an OnMerge Barcode in a Text Box.

Using a Table to Position a Barcode

You can position an OnMerge Barcode anywhere on the page by putting the barcode inside of a table and then formatting and positioning the table as required.

You can place barcodes in any size of table, but use a 1x1 table with no borders if you just want to position a single barcode. After you have dropped a barcode into the table, you can move the table where you need it.

To create Tables:

Use Insert, then Table then Insert Table; it's often useful to then select the table and click Layout, Properties, Options, then check the Automatically resize to fit contents box.

For detailed step-by-step instructions, see this video.