I just erased a SD card using disc utility in High Sierra and now the card is unreadable to my computer or camera. so now what?

If the main use for this card is the camera, put the SD card in the camera and use its format function. Typically, this will be a FAT32 format with an MBR partition map.

I concur with Kurt. The best way to properly format an SD card is in the camera. Put it back in the camera, go to the camera's menu and see if you can reformat it that way.

In Disk Utility, depending on what you did to it...? You may need to select "Show All Devices" in the small "View" option window, in-order to see the SD card and re-format it MS-DOS FAT-32.

What format did you choose for the card, post erasure.