In Mathcad Prime the function WriteCSV does not exist, but WRITECSV does exist, and it accepts no units.
You have to strip them off.
The reason for it not accepting units is probably, to make the user aware that writing out united values to a CSV file (or any other file format that does not work with units, such as MsExcel...) is to be done with great caution. Example matrix with units:
YOU, as a user, have to provide the conversion, so you know how the (unitless!) numbers in the .CSV file are to be interpreted.
The best way to strip them off, is to divide the matrix by the unit that you want them to be:
The data in the .CSV file can be read back immediately after the WRITECSV statement is executed (but the WRITECSV function itself already did it upon execution...)