Proactive Response to Messages

I need help responding to the message below in a proactive way:

The supervisor’s response,
Hi: Sorry for the late response. I was away on a trip to Beaumont.
Unfortunately, no one is available to cover your shift for you. Ram will be working at the prison on Friday, and there is no one else to cover unlike when Sonya was off. Although I will have the car at my house, I need to be available in case it gets crazy and Sonya needs some help in Brownwood or anywhere else for that matter.
It has unfortunately come to the point where I need to explain to the techs that exceptional work is demanded from them as well as hold them responsible for their actions and behaviors. Everyone needs to work together as a team, and this includes you. Complaining so much after we gave you the opportunity to be a full-time team member is not acceptable behavior. I am not trying to scare you or threaten you, but you need start thinking about your future with this company before you complain again about the schedule”.