Since you are trying to send the entire amount in your wallet, use the "Sweep" feature that spends old unmovable amounts. It's on that Send tab in your post (for "unmixable outputs.")
Thanks for the reply, but as you can see the "sweep" option is on and still won't let me make the transaction!
I tried to send another amount (for example 10 XMR), but again it gives me an error.
What to do?