At the end of his letters, Paul often asks that the grace of the Lord Jesus Christ be with the people he is writing to (see also 1:2). Believers need Christ’s grace for both salvation and daily life.
Here, your spirit likely refers to the deepest part of the believers' inner being. The spirit is the part of the person that relates most directly to God. Paul is praying that Christ’s grace would strengthen and sustain them at the core of who they are.