Since "que" is in front of "un", it means "I bought only one bike" (not two). If I want to say "I bought only a bike." (meaning I bought a bike and nothing else), can you say "Je n'ai acheté un que vélo?
This has been asked many times.
Please take a look at my answer to another student. What you suggest is wrong I am afraid.
To write "Je n'ai acheté qu'un vélo" means to me that only a single bike was bought where "un" equates to "a" in English.
In other words, the purchase was a bike only, not anything else.
Hope this helps.
The link from Cécile is what I was trying to find (unsuccessfully). Just to add for clarity - you also cannot put 'que' between an article and its noun; to restrict the noun, 'que' comes directly before the 'article-noun' not between them.
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