I finished the beginner text, and received a certificate of completion.
初級テキスト beginner text/beginner textbook
終えて finish
修了書 certificate of completion
もらった I received/I got
「最近始めた習い事のピアノで一冊目の初級テキストの修了書を先生からもらった」というのは I received a certificate of completion of the first beginner text from my teacher for the piano lessons that I started recently で表現することができると思います。
例文 I received praise from my teacher when I finished the beginner text.
「初級テキストを終えて、先生から褒め言葉をもらった。」
参考になれば幸いです。
"I finished my beginner piano book and got a certificate."
"I finished my beginner piano book and got a certificate."
"My teacher gave me a certificate for completing my beginner textbook."
"I received a certificate after finishing the first beginner-level piano book."
「初級テキストを終えて修了書をもらった」は英語では、
* finish / complete(終える)
* beginner book / beginner textbook(初級テキスト)
* certificate(修了書・認定書)
などを使って自然に表現できます。