The answer is:
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"no error".
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The comma is correctly placed before the conjunction "for"; and in this instance in which the word, "for", is used, a comma is necessary. No other part of the sentence needs—or requires—a comma, and there are no commas that are placed unnecessarily.
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As a side note, I would recommend that the first letter of the first word in the sentence: "Abbey", be capitalized, because:
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1) It is the first word at the beginning of the sentence; and:
2) It is someone's name, which is a proper noun.
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I also recommend placing a "period" at the end of the sentence.
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To read:
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Abbey is most interested in learning how to mount her horse, for she already knows several things about dressage.
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