You are likely trying to enter your purchase Serial Number instead of the activated Registration Code.
If you run a transcription agency, buying individual codes is inefficient. NCH offers site licenses. You pay once for, say, 10 users and receive a master registration code or license file that can be pushed via group policy (GP) to multiple machines. express scribe registration code
NCH registration codes are version-specific. A code purchased for Express Scribe v8.0 will not work on v11.0. You must either download the specific older version you paid for or purchase an upgrade. You are likely trying to enter your purchase