En viitsi odottaa enää.

Because Finnish does not use a separate word like English not. Instead, it uses a negative verb that changes according to the person:

• en = I do not
• et = you do not
• ei = he/she/it does not
• emme = we do not
• ette = you plural / formal do not
• eivät = they do not

So en viitsi literally works like I do not bother / I can't be bothered.

Finnish often leaves out subject pronouns when the person is already clear from the verb.

Here, en already tells you the subject is I, so minä is not necessary.

• En viitsi odottaa enää. = normal, natural
• Minä en viitsi odottaa enää. = possible, but more emphatic, like I am not going to bother waiting anymore

So the missing minä is completely normal.

Viitsiä is a very Finnish kind of word. It does not match one English verb perfectly.

It usually means something like:

• to be bothered to
• to feel like making the effort to
• to have the willingness to do something

So en viitsi often means:

• I can't be bothered
• I don't feel like it
• I don't want to go to the trouble

It is about willingness/effort, not ability.

Good question. The dictionary form is viitsiä, but after the negative verb, Finnish uses a special form of the main verb called the connegative.

So:

• affirmative: viitsin = I bother / I am willing
• negative: en viitsi = I do not bother / I can't be bothered

That is why you get:

• en viitsi not
• en viitsin and not
• en viitsiä

Because viitsiä is followed by another verb in the first infinitive, which is the basic dictionary form.

So the pattern is:

• viitsiä + verb
• en viitsi odottaa
• et viitsi tulla
• ei viitsi tehdä sitä

This is similar to English patterns like can't be bothered to wait, where one verb is followed by another.

Enää means anymore or any longer in negative sentences.

So it adds the idea that this was true up to now, but the speaker has reached a limit.

Compare:

• Odotan vielä. = I am still waiting.
• En odota enää. = I am not waiting anymore.

In your sentence, enää means the speaker does not want to continue waiting from this point on.

They are often a pair:

• vielä is usually used in positive contexts: still, yet, more
• enää is usually used in negative contexts: anymore, any longer

Examples:

• Odotan vielä. = I am still waiting.
• En odota enää. = I am not waiting anymore.

So in En viitsi odottaa enää, enää is the natural choice because the sentence is negative.

Yes. Finnish word order is fairly flexible, and changing it can shift the emphasis.

These are all possible:

• En viitsi odottaa enää. = neutral
• En enää viitsi odottaa. = emphasizes any longer
• Minä en viitsi odottaa enää. = emphasizes I

The version you were given is a very natural neutral sentence.

Very often, yes.

In everyday Finnish, viitsiä is especially common in:

• negative statements: En viitsi.
• questions: Viitsitkö auttaa? = Would you mind helping? / Could you be bothered to help?

Positive forms like viitsin do exist, but they are less common and can sound a bit marked depending on context.

So learners will hear en viitsi much more often than viitsin.

Here are the main present-tense forms.

Affirmative:

• minä viitsin
• sinä viitsit
• hän viitsii
• me viitsimme
• te viitsitte
• he viitsivät

Negative:

• en viitsi
• et viitsi
• ei viitsi
• emme viitsi
• ette viitsi
• eivät viitsi

Notice that in the negative, the main verb stays as viitsi for every person, while the negative verb changes.

These are close, but not identical.

• En viitsi odottaa enää.
  = I can't be bothered waiting anymore.
  Focus: willingness / not worth the effort

• En halua odottaa enää.
  = I don't want to wait anymore.
  Focus: desire / preference

• En jaksa odottaa enää.
  = I don't have the energy/patience to wait anymore.
  Focus: endurance / patience / strength

So en viitsi has a slightly casual, everyday feeling of I just don't feel like bothering with that.