Why is the will to do good present but the ability to perform good not found?
The will to do good is present because the new nature desires it, but the ability to perform is not found because the old Adamic nature has nothing good and keeps tipping the balances against the new nature. The answer to this struggle comes in chapter eight through the Holy Spirit.
Les teaches that Paul is describing the conflict between the new nature and the old Adamic nature. From the old nature's point of view, there is nothing good, so although the will to do good is present, the ability to perform it is not found. The old nature keeps tipping the balances against the new nature's desires. Paul repeats this in verse 19: the good he would do he does not, and the evil he would not do he does. Les notes that the answer to this question is not self-effort or positive thinking, but comes in the next chapter, chapter eight, where the Holy Spirit is mentioned many times, in contrast to only one reference in the first seven chapters.
For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not.