First things first, please make sure you have entered the validation code in all characters. Some tips that have helped out players before: 

Tip 1: The validation may contain upper, lower cases or numbers, sometimes the lower case 'L' may look like the upper case 'I'; and the upper case 'O' may look like the number '0'. If you are unsure what the characters are, simply copy it into a text editor and change the text font. It will normally make it easier to read. 

Tip 2: Some devices' keyboard will automatically attach an empty space at the end of the validation code. Please make sure that you check and remove all redundant empty space. 

If none of the tips helps, please send us an in-game message via Menu > Help, and we will do our best to help out!