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Time Machine backup out-of-date - The last Time Machine backup is over 10 days old.
It appears that your Time Machine backup is out of date. If you need a new backup disk, here’s a few excellent ones sold by Apple:
If you have an iMac (Late 2015) or earlier, or a Mac mini (Late 2014) or earlier, you will also need this cable.
You can then follow the steps in this Apple Support article to setup your Time Machine backup: Back up your Mac with Time Machine - Apple Support.
Also, moving forward, I recommend that you use iCloud in addition to Time Machine to sync all your files and photos to all your Apple devices. Although iCloud is not a "true" backup service, iCloud can also act as an emergency off-site backup, but this is why iCloud should be used in addition to Time Machine. Here are some resources on setting up iCloud for Mac:
After you setup iCloud, turn off "Optimize Mac Storage" in System Preferences > Apple ID/iCloud. That way, all changes made to files in iCloud are also backed up to Time Machine. If you have this setting on due to a limited amount of storage on your Mac, that is fine. Just know changes won't be backed up to Time Machine, only iCloud.
Fusion Drive
However, the reason your iMac is slow is because you have a split Fusion Drive. Follow these steps to correct it: How to fix a split Fusion Drive - Apple Support. This will erase all data, so make a Time Machine backup first and consider using iCloud.
Jack