The most difficult thing when you start your own company is to recruit employees. Here are some reasons to support the idea.
First of all, it’s hard to find eligible people for a startup. Competent people prefer to work in a large company rather than to work in a startup which is usually small.
Secondly, employees in your company may try to get a job offer because there are a lot of tasks due to a lack (the noun "lack" is countable, so it should be used with a determiner like "a") of workers and they may get a low salary ("salary" is also countable).
For those reasons, I think recruiting capable employees is the most difficult factor for a startup.