Hi Karlee,
xPC Target has been replaced by Simulink Real-Time quite some time ago (around 2014, if I'm not mistaken). Simulink Real-Time Explorer is most probably the replacement tool you're looking for (here is the documentation for the version used up to R2020a, and here's its replacement in R2020b and later).
Furthermore, support for custom target hardware has been removed from Simulink Real-Time in R2018a. Starting from that release, only Speedgoat target machines are supported.
For more information, have a look at www.speedgoat.com and at the Release Notes for Simulink Real-Time.