Are you following the instructions here: Install from Source — tvm 0.9.dev0 documentation?
Assuming you have the cmake executable installed, the instructions assume you are working from inside a git checkout of TVM, so:
git clone https://github.com/apache/tvm.git
cd tvm
then you will find the referenced cmake/config.cmake (i.e. this file).
To enable debug mode for relay (i.e. to use TVM_LOG_DEBUG), you must build with the USE_RELAY_DEBUG flag on. So either add set(USE_RELAY_DEBUG ON) to the end of your cmake config or invoke cmake with cmake -DUSE_RELAY_DEBUG=ON ..