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Most likely the motherboard, or more specifically the BIOS instruction set(micro code)are way too old to use UEFI GPUs. Someone else i tried to help, though they didn't listen until after they replaced their motherboard, had all sorts of issues trying to get a new RX series GPU to work on a really old board. If the instruction set isn't there hardware doesn't know how to talk to each other.
You may also e running into power limitations here. A 350W PSU is recommended for systems using a RX 460. Do you know what is inside of that Dell?
There is a section named "How do I fix Graphics and Video Card Driver problems?" in Dell Optiplex 745 Manual in which following information is given to fix the issue that you are using:
Roll back to the previously installed Driver
Device Manager, an inbuilt utility in Windows, allows you to troubleshoot common driver-related errors. For instance, you may sometimes start experiencing driver related problems after you have updated it. To resolve the issue, you can roll back to a previously installed driver. To do this in Windows 7/Vista, perform the following steps:
– Open the Control Panel window and double-click Device Manager
– Double-click Display Adapters to view all devices under it
– Double-click the device that is causing the problem
– Click the Driver tab
– Click the Roll Back Driver button and follow the onscreen instructions