It does sound like a glitch, as you should be able to convert any unwanted prize to get rid of it, even if doing so would reward you with no oddments.
If you can use the Claim Later option, that should at least enable you to close out TH. Of course, you won't be able to use another key until you either claim or convert the prize, as you can't bank it.
Sure, you could subscribe to claim the prize...but is it worth it to you to spend real-world money on membership or over 22M coins to buy a bond to get members, just so you can claim 5M coins? It wouldn't be worth it to me. You have to make that decision for yourself. Maybe you just don't want members. That's perfectly fine. I don't either.
If you choose not to become a member, your only option is to convert your prize to clear it from the TH interface. Try it again. If it still won't convert, I'd say you've encountered a glitch. Either that, or it's just a nasty little trick to try and force you to buy members. If you're still stuck, I'd suggest taking Threes' advice and filing a bug report, either in-game or by posting in the Community Led Technical Support section of the forums. Good luck!
Aslon Dak: Still 'Scaping after 19 years!
You can buy XP. You can't buy experience.
"...From The 'Scape" is very nearly here.
19-Jul-2019 02:15:51