Sometimes your email program may change the text of the license – perhaps to translate it into your language or to add or remove line breaks. Any changes to the license text will make SharpCap reject the license code.
Check the email containing your license – you should see that it has an attached text file that contains a second copy of your license key. Open the text file and try using that copy of the key which should not have been altered.
You can find more help regarding license keys on this page.