Hi and Welcome. My Name is XXXXX XXXXX I look forward to assisting you today with your question.iMac1.sparsbundle is the backup database of your Mac. It contains all your files. Currently your Mac can't access that sparsbundle image either because it is corrupted or your Mac can't simply find the location. So, we can try some basic troubleshooting steps and see if that works or notTurn off your Time Machine backup disk or disconnect it from the Power/USB Cable. Then after 1 minute, connect it back. Open Time Machine, move the slider to off. Turn it back on and then click Select Backup Disk option. Then select your Backup disk and restart Time Machine.
Also, go to Applications > Utilities folder and open Disk Utility. In Disk Utility, select your backup disk in left column. Under First Aid tab, click Verify Disk then Repair Disk. Then click Repair Permission. Let me know if it returned any errors.
Try to backup again. Please let me know if that didn't work. So we can continue troubleshooting.