This refers to the moral law, not the sacrificial law. The sacrifice of Christ fulfilled the requirement of both the moral and sacrificial law, empowering us to fulfill the moral law. But trying to fulfill the sacrificial law would imply one does not believe and accept Christ’s one and only sacrifice for sin.
As well, trying to keep the moral law in an effort to obtain forgiveness is also a refusal to accept Christ’s one and only sacrifice for sin.
It’s quite simple, sin is the breaking of the moral law, and where there is forgiveness of sin there is power over sin, and power over sin is power to fulfill the moral law.
The text says that by the Spirit we walk in righteousness, so the Spirit always causes us to keep the moral law, and love is the fulfillment of the moral law.
People who purposely break the moral law are not led by the Holy Spirit, they are resisting Him, grieving and quenching Him. The love of God is not ruling their hearts.
The text speaks of “fulfilling” and “establishing” the moral law, not being “under” the moral law, as there is a big difference. If you were still “under” the moral law you would be condemned as a law breaker.
Now, as a free man, you are not “under” the moral law, but you don’t cast it away either, you fulfill and establish the moral law by the Spirit, by faith.
Therefore, the person who believes that because they are no longer under the moral law they are not obligated to keep it could have no relationship or connection to the real Holy Spirit, as the text says He always causes us to fulfill the moral law.
(Sabbath days, feasts, clothing, food, etc were all fulfilled by Christ’s sacrifice, they are not in the moral law)