J-Link EDU Mini
On regular 20-pin JTAG connectors all JTAG signals are connected from target device to the debug probe as described here. The J-Link EDU Mini on the other hand uses the 9-pin Cortex-M interface instead.
On the 9-pin connector signal nTRST is not defined. On most target boards it is pulled up but on some boards it is left floating and needs to be controlled by the debug probe.
Should you run into issues setting up a JTAG connection check the state of the nTRST signal. If it is pulled low either try to pull it high through an external wire manually or route nTRST to pin 9 of the J-Link EDU Mini debug interface. In the 9-pin Cortex-M specification pin 9 is usually defined as NC however in case of the J-Link EDU Mini that pin is used as nTRST instead to be able to connect to target devices as well that have that signal routed to their target device.