From a blog post:
"I wanted to write fiction, but couldn’t seem to sell it."
This comma is not necessary. Usually, a comma separates two complete sentences before a "but" or "and." So a comma would be necessary if another "I" appeared:
"I wanted to write fiction, but I couldn’t seem to sell it."
I would recommend no comma if there isn't a second "I," but some might say they like the comma because it adds a pause.
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