The warning is being generated because there is an error randomizing your req item. To provide Can you post the code related to the sequence_item of req?
Also, you are using a randomized int to control a loop count. Be aware that it could randomize to a negative value. Use an ‘int unsigned’ to ensure a positive value.
A failed randomization will normally indicate that there is a constraint solver conflict where the constraints are mutually exclusive. Are there any constraints in your sequence item (axi_rd_transfer)? You have a comment about them but they aren’t shown.
By the way, when you run into problems like this with the uvm_do* macros, you might find it easier to debug if you rewrite them directly. So, instead of
Since you’re now calling randomize() directly, it gives you added flexibility is you want to randomize specific fields of axi_rd_seq_0_1:
axi_rd_seq_0_1.randomize(burst_num) with {axi_rd_seq_0_1.burst_num in (...);};
Personally, I find it easier to understand what the direct code is doing rather than trying to remember what order things happen in with the macros. Give it a shot.
Good luck,
-Tom