Employee with bad attitude?

It's been a while since I supervised staff. There were times when I made a complete mess of it and times when I learned from said messes.

You need to find out why the employee has such an attitude. Then you have to explain why that attitude doesn't work in our profession and then devise a positive action plan that is a win-win for you and the employee.

It may be that there are personal reasons influencing the employee's attitude. You can't prepare for that without having an inkling of what it might be. The best boss I ever worked for expected us to turn up for meetings no earlier than a minute or two before the start time. Then he spent five minutes asking each attendee in turn how life was going. So when I had a serious matter to address in my personal life, I knew I could confide in him and life at work was so much easier.

If it is professional, does the employee understand what a well-done job looks like? They don't teach that in college. Did they, perhaps, come in at too high a level? If so, compensation might be an issue, if you need to realign their duties with their knowledge and experience. Most places just fire those employees, but I have seen a couple of employers try to work with the employee and develop them. One worked and one didn't although, in the latter case, I think the partners pulled the trigger too soon. He was coming along nicely but one partner wanted immediate improvement.

Forgiving or understanding? I think there's a subtle difference.

I believe there is only one way to do a job - the right way. What that looks like will depend on the task. So, as a supervisor, I always held people to account if effort was insufficient to get the job done right. I never gave anyone a hard time over mistakes born of ignorance. Sloppiness or an "I don't care" attitude felt my wrath. I backed it up by holding myself to those standards as well. I was never "popular" but I think I earned respect.

I was an association football referee in Scotland and England for about two decades. At the youth and amateur levels, a mistimed tackle would get the opposition a free kick and the culprit, maybe, a wag of my finger for a first offense. As I progressed through the semi-professional game, I was less tolerant. Mistimed tackles will still happen, of course, but they should be much rarer the higher up one goes and skilled players should not use a mistimed tackle as an excuse for a deliberate foul.

Other things that could be happening are:
a) They don’t agree with something you did and the best thing is to part ways because you will never change and they have a real problem with it. So it’s an ethical issue.
b) They are not in the job they want to be in. For new employees, they might be coming to the realization they hate this career and want to switch to a new career. If you have an experienced employee who is very good at one thing (specialized) but now is learning something new, there will be a complete disparity in their attitude vs your evaluation of them. Result: they start their own business 3 floors down from you. If this happens: renew ties in about 2 years after you they are going to make it. It’s good to have colleagues.
c) COVID made things weird. Some people hated working from home (ADHD) others loved it (OCD). If you have a person who hates working from home, do not ask them what equipment they need. Just start providing it: printer/scanner/fax, desk, softphone app, file cabinet, chair, stapler, staple remover, ruler, paper, mobile appto login, etc.
d) If they are ADHD, make exercise class before work mandatory. That way they show up on time to work.

Of course, your situation is unique and we have no detail about it, but in my experience the above situations cover every situation I’ve run into.

Spend a lot of time guiding and a lot of time listening.

Blame yourself (silently) for anything and everything they do wrong and actively fix it with more coaching, simple procedures, and training. Continuously tell them "what and how".

Be honest and address the attitude itself directly. But ask questions about it in a way that you are genuinely(!) ready to listen as to why an attitude exists.

After a certain amount of time doing this, if you still don't have success, then blame yourself for being a bad hirer (silently) and fire them.