As far as I know, there is no function in ArcGIS Pro or Desktop that can create a network distance buffer centered at a line. Service area function provided in Network Analyst can only create network distance buffers centered at points. The way to create network buffers around a line is to transform the task to create network buffers centered at points and then dissolve. For example, you have a freeway segment, and there are two nodes (original node and destination node) which belong to the freeway. We want to find more nodes along the freeway. The way to find nodes along the freeway is to find the intersections of the freeway segment with other roads. Once you have the nodes along the segment, origin node, and destination node, you create network buffers centered at these nodes, and then dissolve the network buffers.
Like the attached image shows: The red line is the freeway segment, the blue lines are other road segments, the green points are the intersection nodes, and the yellow nodes are origin and destination nodes . We can create network buffers centered at each points and then dissolve the network buffers.