Hi,
This error means that gdb requests the device to read the memory at address 0x20050004 and the device responds with an error. The address would normally be computed by OpenOCD based on the debug settings (e.g. device script) and the ELF file produced by the mbed build.
The best way to troubleshoot it would be to recheck the device datasheet whether it’s a valid memory address (e.g. part of RAM). If it is, please double-check the OpenOCD debug scripts and settings for possible settings preventing memory reads from working. If it is not, please recheck the ELF file and the debug settings for possible hints pointing to the incorrect address.