sync between v.6 and v.7

Welcome to the forum, @Allard! Sync isn't typically affected by having different versions of 1Password (6 vs 7, etc) -- unless one of the versions you're trying to use is really old. What matters is that you use the same sync type. You can upgrade to version 7 on your macOS 10.13 (“High Sierra”) Mac -- but don't create a 1Password account in the process if you're planning to stick with Dropbox. You'll need to download/install 1Password 7 for Mac from our downloads page and purchase a standalone license instead if you want to continue in this way.

Regardless of how you approach it, I have to caution you that OS X 10.11 (“El Capitan") is quite old at this point, and is already starting to have compatibility problems for users who update all other aspects of their software environment. For now, 1Password 6 for Mac works quite well on El Capitan...but since 1Password 6 for Mac has been in legacy mode (not receiving development attention or updates) since version 7 launched in May of 2018, the day will come when version 6 just isn't compatible with other aspects of 1Password you depend on, whether that's changes in the 1Password account servers or changes in Dropbox's APIs or any number of other possibilities. Let us know if you have any questions.