The LED Characteristic (0x1525) appears to be controlling LED 3 for the nordic_blinky example. My thought is that this might be a write only characteristic and so that is maybe why you are getting the error.
As others have pointed out, it is difficult to know with the little information you have shared. A link to the sample code you are following would be a helpful start.
Also, have you tested the nordic_blinky with general tools? For example, in chrome you can go to the URL: chrome://bluetooth-internals/#devices
. It will tell you if the characteristic is readable and writable. Being able to share that would be useful information for others to help. If you are able to read or write in Chrome, then we can be more confident the issue is with your code. Again sharing it working in Chrome and your code that is failing will be helpful for others to assist as to where the issue might be.
Try these experiments and update the question would be my suggestion.