You will need to contact the inmate's counselor and tell them the situation. You will then be able to send the un-signed check to the facility where they will have your inmate endorse the check to be deposited in their commisary/phone account.
https://www.inmateaid.com/ask-the-inmate/my-boyfriend-just-went-to-cherokee-county-adult-detention-center-and-his-last-paycheck-from-his-job-from-before-getting-locked-up-just-come-in-the-mail-how-would-i-go-about-putting-his-paycheck-on-his-books#answer
Accepted Answer Date Created: January 29,2018