Separate this in different rows

The error is thrown because takes an optional argument and the [FV] in the next line is interpreted as such, but the optional argument to should be a valid TeX dimension, which FV obviously isn't. You can put a relax after to make it stop looking for an optional argument:

To calculate market value, can use textbf{calculator}:relax
[2nd][FV]: Clean previous datarelax
[FV] = Face Valuerelax
[N] = Periods in which coupon payments happenrelax
[I/Y] = Market interest rate (YTM) % as noted by egreg, don't put relax on the last line of the paragraph

But perhaps it's a better idea to use an environment like description for this:

documentclass[]{article}

usepackage{enumitem}

begin{document}
To calculate market value, can use textbf{calculator}:
begin{description}[nosep, font=ttfamily]
  item[{[2nd][FV]}] Clean previous data 
  item[{[FV]}] Face Value
  item[{[N]}] Periods in which coupon payments happen 
  item[{[I/Y]}] Market interest rate (YTM) 
end{description}
end{document}

This way you can quickly change the style to your liking with using the enumitem setup.