I need to say: Of course that idiot couldn't buy the tickets after all.
Yappari, ano baka ga chiketto o katte dekinakatta
OR
Yappari, chiketto o katte dekinakatta. Ano baka.
(dekinakatta for past negative tense of dekiru, is this correct?)
Yappari, ano baka
wa chiketto o
kaukoto ga dekinakatta /
kaenakatta
(1) Yappari
It depends on exactly what you'd like to express "of course"
Yappari (colloquial for "yahari") is like "as I expected"
Mochiron is another option if you want to say "as a matter of course".
(2) ga / wa
Unless you want to emphasize that it is that idiot, not anyone else, you should use "wa" i/o "ga".
(3) potential
dekiru = be able
[verb] koto ga dekiru = be able to do ~
Instead of using "koto ga dekiru", you can express potential by conjugating the verb itself.
e.g.
ka-u (to buy) -> ka-eru (be able to buy)
kaku (to write) -> kakeru (be able to write)
aruku (to walk) -> arukeru (be able to walk)
taberu (to eat) -> taebrareru (be able to eat)
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Edit: I just found you posted exactly the same question in a different thread!
You should not double post, please.

(Mods: could you please merge this to another thread "need a word or phrase translated?"?