The source line contains an odd number of quote symbols. Inspect it from left to right and make sure that all literal character strings begin and end with a quote. All literal strings must begin and end on the same logical line. However, you can write long literal strings in two ways. First, you can escape the end of line by making the final character in the line a backslash (however, there is a fixed maximum of 256 characters on the total length of a single quoted string). Second, in the DISPLAY and LET statements, you can concatenate shorter literal strings using the comma. Here are two examples.
Both display the alphabet with no embedded white space.