I'm afraid that your pretty block is not eligible for that particular block exchange. From what we can see in the photo, it looks like 11 inches wide by maybe 12.5 inches long (can't see exactly). The exchange calls for "divisible by 3" and the 11 inches part won't qualify.
The "divisible by three" part means the blocks can be 3.5, 6.5, 9.5, 12.5, or 15.5, etc. Your note doesn't give enough info to tell if the exchange is specific about requiring "squares" vs a "rectangle" like yours. Save this cute block for something else or make a wallhanging or pillow from it!