There are couple of things you need to get into,

1. xml results not getting generated or your azure DevOps unable find that?
2. Is it generating locally?
3. If its generating locally it should generate in build also, so now what kind of agents you are using custom agent or hosted agent?
4. If its hosted agent you need to check where the xml file is placed on, This can be done, Create command line task and which outputs all the directory

a) Create a command line task, set working directory as your source file.

b) put a command in command line task and dir /s execute the build, the build output should display all the directory and subdirectory you can verify xmlfile is generated or not.

c) now you ensure the path to xml file in the publish results tasks are given properly.

Hope I answered your question feel free to reach me on linked in.