Of the four basic tasks of a leader, the second is to prepare the team.
The function here is to assemble (hire or recruit) a team that is capable of doing the things your organization tasks you with, and then to train them as necessary in the core competencies. As team members leave, new members have to be fit into the group and trained to the same level of competence.
Beyond that, though, when you start something new you may need to recruit a temporary addition to the team, or maybe train team members specifically for a new requirement. If you don't have the skills yourself, then arrange for the right person.
The key point: You can't expect anything from your people that you haven't personally verified that they can do. If they don't have the skills, it's not their fault. It's up to you to either verify competency at the time of hire, or to train it into new employees.
It's not significantly different than coaching a sports team. You can either pay the premium to hire proven performers, or you can recruit raw talent. Either way, it's up to you, the coach, to meld the pieces into a unified team that gets it done on the field.
In the real world, this is usually an on-going process, done on the fly as team members turn over. That doesn't relieve you of the responsibility to make sure all team members are capable. Many organizations have formal training programs to make this part of the job easier to keep track of.
You owe your team members the training they need to do their jobs. You owe your organization the effort it takes to field a competent team. And don't forget resourcing: Your team won't do much without the right tools to work with.
Bottom line: When it's time to perform, your team had better be ready. It's the leader's job to make sure they are.