Transitioning from block-based programming environments to text-based programming languages can be challenging as it requires students to learn new programming language concepts. In this paper, we identify and classify the challenges encountered when transitioning from block-based to text-based programming. In particular, we investigate differences that emerge in learners when using a structured editor compared to an unstructured code-authoring editor. We followed 26 high school students (ages 12-16; M=14 years) as they transitioned from Scratch to Python in three phases: (i) learning Scratch, (ii) transitioning from Scratch to Python using either a structured or unstructured editor, and finally, (iii) testing Python code composition skills and conceptual understanding using an unstructured editor. Our results identify 27 distinct types of challenges identified via a thematic analysis of over 400 hours of video captured from the sessions. Our results show that learners who used a structured editor during the transition phase had 4.6x less syntax errors and 1.9x less data-type challenges compared to those that didn’t.