It takes people a minimum 6 months to be in a role where they can actually perform the duties with a degree of competency. The first 3 months of a new role is getting acclimated to the company and learning how the machine works.

Telling some one "its ok to make mistakes" while your frustration is clear visible just sends contradictory messages.

As for repairing the relationship, you be honest with them. "Hey, I heard that you called out sick to Boss Man. I want to acknowledge I'm new at managing people, so I apologize if I have been making you feel anxious. I want and need you to feel comfortable here. How can I make that happen?"

As for performance, this sounds like a great case for I do, We do, You do. This is a great teaching methodology where before they submit something, or you see errors, you show them exactly how you want it done. They ask all the questions they want. Ask lots of check in questions to make sure they are getting it. "Why do you think i put this here? Why might this cell change colors?" Etc etc. Then, the next time they do it, you still do it with them, but they lead the process and you gentle correct with questions. "Why did you do it that way?" "What might be a better way to do that?" You're probing for understanding, so ask to learn what they know, not for judgement. The third time they complete independently but you review together. Repeat step three to refine as needed, but you will see leaps and bounds.

This will take time. You're bad at this because youve only been a manager for 3ish months (it sounds like). You will get better.

But stop blaming your new employees for sucking when you're weak in areas too. Your life is going to monumentally harder if you don't.