Jag behöver plåstret nu, men det ligger inte i väskan.

Plåster is the basic (indefinite) form meaning a bandage / a plaster.
Plåstret is the definite form meaning the bandage / the plaster—i.e., a specific one you both have in mind (the one you’re looking for).

It’s the definite singular ending for many neuter (ett-) nouns.

• ett plåster = a bandage
• plåstret = the bandage

So -et here functions like English the (but attached to the noun).

You learn it with its article: ett plåster. That’s why the definite form is plåstret and why the pronoun later is det (not den).

Det refers back to plåstret. Since plåster is an ett-word (neuter), you use det to mean it.
If the noun were a en-word (common gender), you’d typically use den instead.

Yes, ligger literally means lies, but Swedish often uses it for an object being located somewhere, especially when it’s thought of as “lying/being placed” in a spot. In everyday Swedish, ligger is very common for “is (located)” with things.

Sometimes, but it can sound less natural depending on context. For location, Swedish often prefers a “position verb” like:

• ligger (lies/is located)
• står (stands)
• sitter (sits/is attached/is snugly placed)

Det är inte i väskan is understandable, but det ligger inte i väskan is often more idiomatic for “it’s not in the bag.”

In a main clause, Swedish follows the V2 rule: the finite verb comes early, and inte typically comes after the finite verb.
So you get:

• Det ligger inte i väskan. (verb ligger before inte)
  Not: Det inte ligger... (that word order is generally wrong in a main clause)

After men you start a new main clause, and Swedish again uses V2 word order. That’s why it’s:

• ..., men det ligger inte i väskan.

You don’t invert anything here because the clause begins with the subject det.

It’s common (and often recommended) to use a comma before men when it joins two full clauses:

• Jag behöver plåstret nu, (clause 1)
• men det ligger inte i väskan. (clause 2)

It’s not always mandatory in informal writing, but it’s a standard, clear choice.

i means in (inside).
väskan is the definite form meaning the bag (a specific bag).
So i väskan = in the bag.

If you meant “on the bag” (on top of it), you’d use på instead.

En väska means a bag (any bag).
Väskan means the bag—likely the one you’re carrying or talking about. Swedish uses the definite form a lot when the object is known from context.

Nu is flexible, but placement affects emphasis. Common options include:

• Jag behöver plåstret nu. (neutral: I need it now)
• Nu behöver jag plåstret. (more emphasis on “now”)

In your sentence, Jag behöver plåstret nu is a very natural, neutral choice.