How to let FPGA get access to HPS pins

Have you got the ARM processor up and running?

LoanIO only works if the processor is running because the system control registers must be configured by software running on the processor in order to allow the LoanIO interface to run.

You'll need to generate a design which include the hard processor system with the require LoanIO parameters. From there you will need to generate a BSP and compile a preloader. Once you have the preloader up and running you'll need a simple bare metal program of an infinite loop resetting the watchdog (if enabled) to stop the processor crashing. The preloader will use the BSP information to set the system control registers to the required.

The reason RX works is that the ports default to input which means you can get data in without the processor running. However for TX you need to get be able to set the TX pin to be an output which can only be done once the processor is running.