@robertanema - this isn't the forum to do technical support, but here's some pointers to maybe get you in the right direction.
1) Repair permissions is still there in El Cap, it runs in the background, just not visible thru Disk Utility. That repair permissions function mainly repairs SYSTEM permissions, operating system files, who can execute, etc. The repair permissions step in Onyx invokes the same macOS repair permissions, it is identical, Onyx makes it easier to execute explicitly. From your description I think you are trying access files in Finder, I have to guess these are not system files, but just user type data files.
2) it sounds like you have an issue with USER permissions, i.e. files that aren't currently owned by the user account that you are logged in as. Are the files you want to open from another Mac that are from a different user log in, or the same Mac, but under a different account? If so, then you either you have to log in as the original user who created/owns the files, or change permissions to allow you, a different user, to access them, so go to #3
3) Are you logged in as a user with Administrator rights? You need to have Administrator rights to give yourself permissions to user files that aren't created by your account originally. See System Prefs/Users and Groups, see if your account is Administrator or Standard. If you are an Admin go to next step.
4) Do a Get Info on the files/file/folders you want access to. scroll to the bottom, Click the padlock at the bottom to unlock. You will be asked to enter an Administrator login and password. If you aren't, or don't know an Administrator login or password on this machine, stop, you can't go further. Once unlocked, select the user/group and change the privilege to whatever you want.
5) if it says you already have Read/Write, try creating a new user on that Mac with Admin rights, log in as current user that's giving you problems, give that new user read/write access to the file, then log in as the new user and see if you can access. If so, the solution may be to just transfer all your data to a new account and give that new user access.
Last resort, take it to the Apple Store or Geek Squad or Mac expert in your local area. Good luck