Problem-solving is one of the 21st-century skills. However, students still have difficulty solving sequences and series word problems. The purpose of this research is to analyze students' errors in solving sequences and series word problems based on problem-solving steps of Polya. The research method is descriptive qualitative. The research subjects were six students of XI-B SMA Plus Ar-Rahmat Bojonegoro who were given written tests and interviews. The written test consists of two sequences and series word problems. The results show that the percentage of students who made mistakes in the step of understanding the problem is the smallest and in the step of looking back is the largest. The research findings show that there are four types of student errors; misunderstanding the meaning of the keywords, incorrectly relating what is known and asked to the previous knowledge, incorrectly distinguishing concepts and strategies for solving real-world context problems due to positive interference, and miscalculation due to pseudo-covariational reasoning from "wrong" answers. [For the full proceedings, see ED630948.]