The thumbnail photo is not the problem, I tried to put the default image that the mod editor uses and the problem persists.
Related: AOE4 Client doesn't let me to publish any AOE4 Mod anymore, Error for Submitting AOE4 MOD, Error Code: C26T-9995R01X000 414D50620131, Please Need Help to Fix this Problem, thank you very much - #9 by Sluagh