Unterminated multiline string
You omitted the vertical bar (|) that marks the end of a multiline string; or you omitted the close brace (}) that marks the end of a multiline string; or you used a brace as one delimiter and the "|" character as the other. For example:
Print |Hi,
Martin.
' ... ' Illegal because there is no matching vertical bar
Print |Hi,
Martin.} ' Illegal because the delimiters don't match.
Check for any unpaired or improperly paired multiline string delimiters and enclose the string appropriately.