how to select which gems to use when buying from reserve

[Alice TW]

Hello hope someone can help. I am enjoying learning Splendor; have only recently started to "reserve" cards to buy later. On a certain game Splendor #249882836 I had reserved 2 cards; when i wanted to buy one of them I am sure I had more than enough of the right colour; not even sure if I needed to use either of the 2 "joker" gems I had received . I think it said I had to choose which gems to spend, but whatever I did I could not get it to activate and purchase . tried a couple of times; explained to my opponent that I was having difficulty with the purchase but he/she still kicked me out when i was only a short time over the time! CAn someone explain what I was doing wrong and/or the EXACT sequence of what you need to click on so that I don't risk this happening again and losing more reputation points! thanks.