Use of an illegal line label or a required line label or line reference was not specified.
Let not used, but required. (i.e. If X then fndo_thing)
Remedy:
Remove all illegal characters from the error-causing line label; line labels must begin with an alphabetic character and may not exceed 30 characters. If a line label or reference is missing, add the appropriate reference.
Add a Let to your statement. (i.e. If X then let fndo_thing)