SSD Doesn't show up on Mac

Hello - I hope someone can help me. I have mac mini late 2014, with Mojave installed - 10.14.6.

I’m gonna list what happened:

  • I got a Sandisk Extreme Portable SSD 500gb and I tried to install it.
  • I went to Disk Utility, erased it, renamed it, and set it to Mac OS Extended (Journaled), GUID etc etc then turned off the mac.
  • I turned it back on pressing Option to pick the SSD and install the OS, it kept reading it as “Windows” and not loading(??).
  • I restarted with normal mode, went back to Disk Utility did everything again and everything seemed okay with the SSD.
  • I turned off the mac, unplugged the SSD and plugged it back in, turned it on again, pressed ALT key, now it doesn’t even show up anymore.

I see it in Disc Utility, in Finder and also when I go to infos. But it doesn’t show up as bootable at startup.

I really need to install this for work, and I need to install the OS on it…