Participants register to attend the Google Calendar event via Google Form
The specified syncing element (Drop-down) turned off "Required". Then, one of the form's submitters does not choose any slot causing the error.
Your customer entered an incorrect email address when submitting the Form. Please use the "Collect email" function or enable the validation feature for the current email field.
Saved dropdown items (or choices) may be deleted or changed to another type.
Saved dropdown items (or choices) may be deleted.
Saved dropdown items (or choices) may be changed to ... type.
unwanted action