What do you see if you type 's into a new document in a plain old ordinary text frame using the same font?
usually it happens when at the font's symbol table is no glyph
Cannot image a font without an 's' character, though I have seen plenty without a typographical apostrophe.
It may be that the font doesn't have either of those characters and so uses what is called the .notdef character (i.e., not defined). But it may also be the encoding of the merge file and/or the use of commas and apostrophes in the file and they are "unbalanced."
If the font has those characters (what font btw) and the CSV file's encoding is UTF-8, UTF-16 or even ANSII, and you believe the apostrophes and commas are balanced, try exporting as a tab-delimited file and use that as a data source on a copy of your file.
Thank you all sorted, looks like it was more of a font issue. Thanks again