What tense is "fuera" here?

What tense is "fuera" here?
 

I would agree if the plot was more complex.
Estaría de acuerdo si le argumento FUERA más compleja.

Subjunctive Imperfect?
if so, why would the Subjunctive be required here?

"Fuera" is preterite subjunctive and is used here because this is how we build a second conditional. We call it "condicional irreal". The condition expressed has not been fulfilled in the past or is improbable in the present or future.

Estaría de acuerdo si el argumento fuera más complejo. (Please note the corrections, the first one must be a typo, but the second one is important: "argumento" is a masculine, so "complejo", which is an adjective related to "argumento" must agree in gender and number with the noun.)

Some more examples:
- Si pudiera, te daría un abrazo.
If I could, I'd hug you.
- Si mi padre no estuviera enfermo, lo llevaría al mar.
If my father weren't ill, I'd take him to the sea.
- Les compraría un helado si no hiciera tanto frío.
I'd buy you an ice-cream if the weather weren't so cold.

thank you.
indeed my errors and typos.

My exercise was actually:
I would agree if the plot was more complex.
Estaría de acuerdo si el argumento FUERA más complejo

But hoping to get an explanation on that tense of "Feura" I pluggedthe English phrase into Google Translate which spat back:
"la trama" instead of "el argumento" for "plot"
and then I tried to correct it manually and my French got in the way and I forget to correct "Complega"

phew.

but thank you.

Now let me try to undertstand this use of the subjunctive.

No worries. :)

Ask all questions you need. This is a complex structure in any language. :D

Something else to consider about your original English text, "I would agree if the plot was more complex."

When I was in high school in the US during the 1970s, my English teacher would have corrected this sentence to: "I would agree if the plot were more complex."

It turns out that this is an example of how the past subjunctive used to work in English. Using "was" instead of "were" has become more common and more accepted as correct since then.

I have a lot of "issues" with "Mango Languages", the learning service I started working working through during the Pandemic.
I write them frequently with questions about things I think are wrong or unhelpful. (like expecting beginners to learn - or understand - these very complicated sentence structures. they seem to just want learners to memorize them by rote.)
but they never change anything or act on my suggestions. which is frustrating.

but this service is free through my local public library and so I am going to press on and at least complete the course.
next step - a one month intensive programme in Costa Rica I hope.

There is a method of teaching complicated language usage via common use and memorization of dialogues. Sometimes the analysis of the usage is more alienating than just presenting common usage of complicated language rules in everyday speech repeatedly.

An English example would be the use of the auxiliary verb do as a means of asking a question. Perhaps the logic of it can be taught, but for some, just plunging into it with examples, dialogues and even memorization is simpler and more effective.

@Pstock: The idea is not exactly to memorize, but to learn to use in context, just like we learnt our native languages. I'm not sure that works all the time, but it's better to learn this way when structures don't correspond from one language to the other.
Don't despair and just ask for explanations or examples. It will make sense as you use them and practice. ;)