If I have no choice but to adopt the one-group posttest-only design, are there any tips for me to solve the problem of no baseline scores?

You are welcome! =]

Thank you.

One possible solution is to ask the participants to recall their previous experiences in the posttest. By including retrospective questions, you may be able to obtain baseline scores. However, there may be a certain degree of recall bias, and you need to develop strategies to minimize the bias