Did you replace the PSU in the computer from a 300 Watt PSU to a 700 Watt PSU?
Here are the specs for your HP: HP Pavilion HPE h8-1017c Desktop PC Product Specifications | HP® Customer Support
The new RX5500xt requires a 8 pin PCIe GPU Power cable to be connected from the PSU to the GPU. (depending on the manufacturer how many GPU Power cables are required.)
Do you have that GPU Power cable connected to the RX5500XT?
Also according to a PSU Website the RX5500 requires a minimum PSU Wattage for your computer of 430 Watts with that GPU card installed.