أَنَا نَاثِرٌ لَا نَاظِمٌ
I write prose, not poetry.
The speaker first affirms prose composition, then uses coordinating lā to deny poetry as the competing description. If the hearer believed that both activities applied, the statement removes that assumed combination.
This lā is not the exception marker “illā”; it coordinates a denial with an already stated affirmation. The analysis therefore depends on the whole construction, not the presence of the negative particle alone.