The phrase is very ambiguous. 'Cum' might be a misspelling or an abbreviation that is not common. If'mom let me come in her stories' is what is meant, it could imply that the mother allowed the speaker to be included or be a character in her stories. But again, this is just speculation without further details.
It could be that the file format of your story is not supported. Make sure it's in the correct format as specified by the upload requirements. Also, check if the size of your story exceeds the allowed limit.
One way to interpret it could be that'mom let me come in her stories' means the mother allowed the person to be incorporated into the stories she was creating. For example, if the mother was writing or telling fictional stories, she might have let the speaker be a character in those stories.
It could be a technical issue. Maybe there's a problem with the game files. Try verifying the integrity of the game files if you're on a PC. This can often fix problems where the game isn't behaving as expected, including issues with story replay.