gasu no konro ga tukaenai node, konya ha kantanna ryouri sika dekimasen.

This is very common in Japanese.

The main sentence ends with できません, so the overall sentence is polite. But the reason clause before ので often uses the plain form:

• ガスのコンロが使えないので、今夜は簡単な料理しかできません。

This is natural Japanese.

A more polite version of the first part is also possible:

• ガスのコンロが使えませんので、今夜は簡単な料理しかできません。

That sounds more formal or stiff. In everyday polite speech, using plain form before ので is very normal.

No. Here 使えない is the negative potential form of 使う.

• 使う = to use
• 使える = can use / be usable
• 使えない = cannot use / is not usable

So コンロが使えない means the stove cannot be used.

It is not the passive form. The passive would be:

• 使われる = is used
• 使われない = is not used

So:

• 使えない = cannot use
• 使われない = is not used

Those are different.

Because が marks the thing that is in the state of being usable / unusable.

In potential expressions like 使える / 使えない, the thing that can or cannot be used is often marked with が:

• このペンが使える。 = This pen can be used.
• このペンが使えない。 = This pen cannot be used.

So:

• ガスのコンロが使えない = The gas stove cannot be used.

For English speakers, this can feel strange because English might treat stove more like the object of use, but Japanese often marks it with が in this kind of pattern.

Here の links two nouns:

• ガス = gas
• コンロ = stove / burner / cooktop

So ガスのコンロ means something like a gas stove or a stove that uses gas.

This の is not possession in the English John’s book sense. It is more like a descriptive link between nouns.

Also, many speakers would simply say:

• ガスコンロ

That is very common and may sound more natural in many situations. So both of these are understandable:

• ガスのコンロ
• ガスコンロ

Both ので and から can mean because, but they have a different feel.

• ので sounds softer, more explanatory, and a bit less direct.
• から can sound more straightforward or assertive.

So:

• 使えないので... = since it can’t be used..., because it can’t be used...
• 使えないから... = because it can’t be used...

In this sentence, ので fits well because the speaker is calmly explaining the reason for the situation.

は marks 今夜 as the topic:

• 今夜は = as for tonight / tonight

This sets the frame for what follows:

• 今夜は簡単な料理しかできません。
  = As for tonight, I can only make simple dishes.

Using は can also give a slight contrastive feeling, like at least for tonight this is the situation.

If you used 今夜が, it would sound very different and would usually not fit this sentence naturally.

Because 簡単 is a na-adjective.

Na-adjectives use な before a noun:

• 簡単な料理 = a simple dish / simple cooking

Compare:

• 静か → 静かな部屋
• 便利 → 便利な道具
• 簡単 → 簡単な料理

So 簡単の料理 is incorrect.

しか means only, but it is used with a negative verb.

So:

• 簡単な料理しかできません

literally has the structure:

• nothing except simple dishes can be done

Natural English:

• can only make simple dishes
• can make only simple food

A key point is:

• しか
  • negative verb

Examples:

• 水しかありません。 = There is only water.
• 日本語しか話せません。 = I can only speak Japanese.

So in your sentence, しか emphasizes limitation.

Normally, しか is used with a negative form.

So this pattern is standard:

• Noun + しか + negative verb

Examples:

• 一つしかない。 = There is only one.
• これしかできない。 = I can only do this.

If you want to say only with a positive verb, you usually use だけ instead:

• 簡単な料理だけできます。

This is grammatically possible, but it does not sound the same. しか...ない usually feels stronger, with the nuance of nothing except...

So:

• 簡単な料理しかできません = I can do nothing except simple dishes.
• 簡単な料理だけできます = I can do simple dishes only.

The first is more natural here.

Because しか usually replaces the particle that would normally follow the noun.

So you do not say:

• 料理をしかできません ✘

You say:

• 料理しかできません ✔

This is very common with しか.

Also, with できる, Japanese often does not use を the way English speakers expect. Depending on the sentence, you may see:

• 日本語ができる
• 料理ができる
• 料理しかできません

So the structure here is natural.

Yes, 作れません could also work.

In this sentence, できません means something like cannot make / cannot manage / cannot prepare. With 料理, it can refer to what kind of cooking is possible.

So:

• 簡単な料理しかできません = I can only make simple dishes.

A very similar alternative is:

• 簡単な料理しか作れません。

That may feel a bit more directly like I can only cook/make simple dishes.

So both are possible, but できません is fine and natural here.