The most common causes, in order of likelihood:
Employee is not in the right team
Someone can be on the service team and occasionally work in the kitchen, manually add them to the kitchen team via their profile. Without team membership, you can't schedule them.
Regardless of where someone has their contract, they can be part of all teams of all locations within your restaurant group. You don't have to offer a new contract of create a new employee when someone works shifts at other locations.
Want to know what to invoice your other location when you have lent out an employee? Go to Reports > Lent.
Want to know what lending an employee of another location costs you? Go to Reports > Borrowed.
They haven't been offered a contract yet
You can schedule employees that have been offered a draft contract. It doesn't need to be signed yet, but it should at the very least be offered to them. If you havent offered them a contract, you can't schedule them yet.
If you will never offer them a contract, but still want to schedule them, you can add them as a freelancer. This could be handy with for example other co-owners that work shifts as well, but send a management fee invoice instead of being employed at the restaurant.
You just added someone to a team
Right after being added to a team, there's can be 1-2 minute window before someone is schedulable. Refreshing your page can help.