In Galatians and Romans, The Apostle Paul is discussing an essential Theological point.
Can we live under the Law and The Spirit at the same time?
Galatians 5 and Romans 13 teach that we are to live under the Law of Love, and love fulfills the Law. For example, if you love someone you will not steal from them, you will not kill them, you will not commit adultery.
This is why when asked about the Law Jesus replied, ‘Love the Lord your God with all your heart and with all your soul and with all your mind.’ This is the first and greatest commandment. And the second is like it: ‘Love your neighbor as yourself.’ All the Law and the Prophets hang on these two commandments.”
The Quote "Perfect Love casts out all fear." means that if you have perfect love, you will have no fear of breaking the Law.
What Jesus is saying is that we have a new Law, and if we live by this Law, we won't break the Mosaic and Rabbinical Laws that came before Jesus.
So, the answer is that we cannot both live under the law and the Spirit. When we live under the Spirit, we live under the new covenant of love.
If you have any questions regarding this quick teaching, please post a question or reply.
God Bless you!