Hi @Nagendra,
These type of questions are outside of the scope of the PYNQ support.
I suggest you familiarize yourself with AXI4-Stream and how the DMA works. As you will learn from the link below, TLAST signals the last byte of the stream so it cannot be asserted all the time.
A good starting point here How-AXI4-Stream-Works
You can also review Cathal’s tutorial Tutorial: using a HLS stream IP with DMA (Part 1: HLS design), you can add ILA in that design so you can learn how it is working.
Mario