In "Word Focus" Glenn McClure writes:

In verses 18 and 21, we have another important word that helps us to see the struggle of the believer — to will is present with me (v. 18), and evil is present with me (v. 21). The Greek word for is present with means “to lie beside” or “to be near.” It can be defined also as “to be present, at hand.” What a paradox! The “will” or intent to do good is present or at hand, but to “perform” (carry the desire into effect) is not present. In other words, the desire to do good was possible, it was in his power, and it was his habitual inclination and purpose of mind to do right. But in contrast, verse 21 also says that evil is present with him. This “evil” is as near at hand as the desire to do good. It raises its ugly head unbidden and undesired, to turn one from doing good. Thus we see here man with sin dwelling (or making its abode) within him and causing him to miss the design of doing good. Even when the will to do good is present, the power of sin is also present. There is only one Power that can enable man to overcome the power of sin — that is the indwelling power of the blessed Holy Spirit.

Source: Studies in Romans: Adult Teacher's Insights, page 34.