Let employees know exactly what they need to do in the future to reach the desired level of performance if required." 1) Should it be "in the future" or "in future"? Yes, either. Almost the same exact meaning in this sentence. "in the future" means at any time after now. "in future" brings in a sense of it having been different, not done, not the required level of performance, in the past. So this does not go so well with "if required". 2) Have I used "if required" correctly in the sentence? I have put it at the end. It works, if you mean "tell them, if required". "If required, let employees know exactly what they need to do in the future to reach the desired level of performance." would also work and not have any doubt about what the "if required" connects with. 3) Is it understandable? Are there any grammar mistakes? It is great.

"Let employees know exactly what they need to do in the future to reach the desired level of performance." This is natural and understandable. Adding "if required" at the end is unnecessary and confusing. What meaning did you intend by adding it to the sentence?

Adding to the other answers, my sense is that “in future” is more commonly used in British English, but rarely used in the US. In the US, “in the future” is much more common.