apps can't installed

It appears the application you want to install requires 10.12 as a minimum. If you want to download that application, you need to upgrade your OS if you can. If you can run Sierra, you can run High Sierra.

Check the computer requirements shown in each link to see if your computer is capable of running that OS version.

How to upgrade to macOS Mojave - Apple Support

How to get macOS High Sierra   

Check to make sure your applications are compatible. .       

Application Compatibility

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

Microsoft Office 365 and the single purchased Office 2019 for Mac now require macOS 10.12 or later for installation. Once Catalina (10.15) is released this Fall, Microsoft may require High Sierra (10.13) or later as they are mapping to Apple's operating system support cycle.

If your Mac can support Sierra (10.12), then it can also run High Sierra, and I would recommend the latter. You will need to verify that your third-party vendor applications (either the vendor site, or roaringapps), printer, and scanner drivers are compatible with High Sierra — before you perform the upgrade. See How to upgrade to macOS High Sierra, which also contains a link to High Sierra in the Mac App Store.

I don't understand your post. What isn't downloading? More information would help.