Always create a copy of your save files ( save-1.bak or just a folder copy) before editing. A corrupted save can crash the game.
If the standalone save editor doesn’t work, many developers suggest using the Ren’Py console ( Shift+O in many games) or inserting a temporary label in the game’s script to adjust variables directly. renpy save editor github link
This usually means the visual novel was built on a heavily modified or highly modern version of Ren'Py that uses advanced encryption or custom Python objects. Try switching from a web editor to a desktop Python-based editor, or check the GitHub repository's "Issues" tab to see if a fix is available. Always create a copy of your save files ( save-1