I think this has come up before, and the reason was that Sublime is sensitive to the format of the text that you give it and sometimes the license email gets the formatting a bit wrong.
If the format of what you’re pasting isn’t similar to the below, that might be the reason.
----- BEGIN LICENSE -----
Your Name
Single User License
0000-0000000
00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000
------ END LICENSE ------