Trying to install a DROP BOX APPLICATION for my MacBook Air - operating system OSXEl Capitan Version 10.11.6 - can I do so with this version?

It should not install if it is not compatible. However, check with DropBox for system requirements of their software. It isn't an Apple product.

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These forums only provide help with Apple products. Thank you for your understanding and support.

According to Dropbox, 10.10 or later will work. See: https://help.dropbox.com/installs-integrations/desktop/system-requirements#mac