I Can't Buy an Engagement Ring!

HELP! I CAN'T BUY AN ENGAGEMENT RING!!!!

I am on level 10 and am on the GET ENGAGED! task, i have made my sim's realtionship to partner and there is the PROPOSE button, i tapped on it but all of the engagement rings greyed out! I can't buy any even if i can afford to, please somebody help me!

(Ps : My sim have more than one partner.)