I'm having trouble starting my back up
It's been a while since I've backed up, trying to use TimeMachine
Had an error last time that said "sparse bundle couldn't be accessed"
is this blocking it somehow?
Question marked as
If it's "been a while", erasing the drive and starting over is likely the best approach. The data is old anyway as you admit you're not backing up. Erase and reformat a storage device in Disk Utility on Mac - Apple Support
Back up your Mac with Time Machine - Apple Support
What in the referring article did not work for you? You have to follow the steps precisely.