I see the bug, thanks for reporting!
Here's fixed save file: https://drive.google.com/file/d/1iDxRRKWhX-4TWjnDbKsEUoXKD1LlNHov/view?usp=shari...
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If anyone else is having this issue and doesn't want to wait for the next update:
This can happen if the center of your single that ranks #1 in the charts graduates before you continue the main story.
You need to open your save file with a text editor and:
- Find an idol with "Variables": ["center"]
- Remove "center" from her and add it to any other idol who hasn't graduated yet
