Im the sole user/owner of my iMac but don't have all administrator rights. Why?

If everything else is Ok and you have all your files in the "owner" account, then you can use the instructions in this Apple article to change the user name of the account. Make sure to carefully read & follow all of the instructions without missing a step.

Change the name of your macOS user account and home folder - Apple Support

Have you tried adding an additional admin-user, so you can verify your account and granted rights?

Here is how to set up a new account: Add a user or group on Mac - Apple Support

Make sure you set it up as an administrator.

Then you should be able to modify other users as well.

Also here is a tip, I've got from an Mac-admin and friend:

When you set up a new Mac set up the first Accounts as an Admininistrator (think of it like it is a Master Account) and set up an additional account (even if it is an admin-account as well).

Then keep the Master account as a fallback and only use it in case of ermergency. Then use the second account as your "regular account". Doing this, you will always have an account, that can manage all other accounts.

So if your regular Account gets corrupted, you can always use your Master account to handle other Accounts.

I've done this for years now and actually only used it once - but it was worth having it :D