Graphics.SetRandomWriteTarget() is not supported with URP Render Graph. This lack of support is by design, because on some graphics API these buffers share the same binding slot-space with other type of buffers. So setting them globally can generate rendering issues in SRP.

It is recommended to implement a URP Scriptable Renderer Feature (custom pass) with an async GPU readback instead. You can implement it within the render function of your Render Graph pass:

using (var builder = renderGraph.AddUnsafePass<ReadbackPassData>("ReadbackPass", out var passData))
{
builder.AllowPassCulling(false);

// Which buffer to read from
passData.bufferHandle = m_OutputBuffer;
builder.UseBuffer(passData.bufferHandle, AccessFlags.Read);
builder.SetRenderFunc(static (ReadbackPassData data, UnsafeGraphContext ctx) =>
{
ctx.cmd.RequestAsyncReadback(data.bufferHandle, (AsyncGPUReadbackRequest request) =>
{
var result = request.GetData<int>();
Debug.Log(string.Join(",", result));
});
});

Check URP Render Graph documentation and URP Render Graph samples for more information. We are planning to include an async readback sample in the coming month.