Final answer:
The error in the bolded sentence is a pronoun-antecedent agreement error.
Step-by-step explanation:
The main grammatical stylistic error in the bolded sentence The joys a pianist receives when they accompany a choir of singers can make her years of practice and lessons all worthwhile is a pronoun-antecedent agreement error. The pronoun 'they' does not agree in number with its antecedent 'a pianist', which is singular. To correct this mistake, the pronoun 'they' should be replaced with a singular pronoun such as 'he' or 'she', ensuring that the pronoun matches the singular antecedent 'a pianist' in terms of number.