Sorry!. Must have gone to bed before you posted the additional information.
In my QuickTime Player 7, all the "PRO" features are grey in the menus. To enable the "PRO" features, at one time long ago, I entered the license key. There is a menu selection "QuickTime Player/Registration...". But now, when I choose it, absolutely nothing happens. A panel should open with a box in it to type my key. But nothing. Nada. Zip. I am in QuickTime Player 7. "About QuickTime Player" tells me I'm in version 7.6.9.
I have used QuickTime Player 7 in High Sierra, Sierra, and El Capitan. It works in all three. The problem is that, after finally having been completely fed up with the bugs in Sierra and High Sierra, I put together my old El Capitan. Life is good now except that obviously QuickTime Player 7 didn't "migrate" properly. My other 163 applications did.
If you indeed have QT Player 7 v7.6.9 installed on a platform running under El Capitan, then that is your problem. Mac QT versions greater than 7.6.6 (i.e., up to v7.7) were only released for use on systems with Mac OS X v10.5 (Leopard) installed. Mac OS X v10.6 (Snow Leopard) was initially released with QT Player 7 v7.6.3 and was later upgraded to v7.6.6 (when Mac OS X v10.6.3 was released) and remains the latest available version of the QT 7 app compatible with systems operating under MacOS v10.6 thru v10.13 which have dual QTX/QT7 MacOS structure embeds. I.e., THESE ARE SPECIAL VERSIONS OF THE QT PLAYER 7 GUI.
Since the older Leopard version of the QT Player 7 "registration" window is linked to and accessed via the global "System Preferences" window and post Leopard (dual QTX/QT7) versions of MacOS X use a separate, individual "local" preference pane, it is little wonder that "nothing happens" when you choose the registration preference option. (I.e., the registration window is, in effect, "orphaned" since the original target destination for the registration routine is no longer valid on an incompatible operating system.)
I suspect your main problem is some confusion regarding the QT 7 version numbering. I.e., the version numbers are "in oder" when it comes to their order of release but are "out of order" when it comes to compatibility with specific operating systems. Not even sure how you managed to install an incompatible combination of QT7 and Mac OS X since normal workflows usually won't allow you to install a "Leopard" version of QT 7 on a post-Leopard operating system and installing a newer operating system on a Leopard operating system normally updates the QT7 version for compatibility on an automatic basis. Have you been moving copies of the QT7 app manually between systems or overriding installer operations with Pacifist or Finder "Show Package Contents" routines?
If you are really using El Capitan on a QT7 v7.6.9 "problematic" platform, recommend you download and install this version of the QT Player 7 app as previously indicated in my "need to reinstall the software package making sure it is the correct version for your operating system" comment.