Incorrect ejection could damage the storage media. That's why the SD card becomes unmountable after you suddenly unplug it from your Mac while erasing in Time Machine.
When you find the SD card or other external hard drive not mounting on Mac, you can try to manually mount a disk on Mac via Disk Utility's Mount button, Terminal commands, and FSCK.
Given that you received the error messages "First Aid process has failed" and “File system verify or repair failed:(-69845)" what you can do next is reformatting the drive to make it mountable again, but before that, you can first recover data from unmounted SD card if there is no backup.
iBoysoft Data Recovery for Mac can help to recover data from not mounting external/internal hard drives on Mac. You can free download, install, and scan the SD card for deleted/lost data, filter and preview the scanned results, and then recover desired items.
