You probably have the LR 5.7 creative cloud trial downloaded and installed, the Perpetual License edition will not work to activate the creative cloud version.
First uninstall the version of Lightroom from your computer using the normal procedure for your opersting system. This will not interfere with your Catalog, Previews, Preferences or other personal files, just the application will be removed.
Download the latest Perpetual version from the link below. Install and enter the License Key and every thing will be fine.
If this has satisfied your question completely , you can mark the thread as answered.
Now it won't let me in develop mode though??
Have you downloaded the perpetual version and entered the License Key?
If so what was the outcome was the key accepted?
If not was there an error message?
If so please quote it exactly.