Kvitteringsnummeret står også under strekkoden, så jeg tar bilde av det.

Because Norwegian very often makes compound nouns by joining words together.

Here, kvitteringsnummeret is built from:

• kvittering = receipt
• a linking -s-
• nummer = number
• -et = the definite ending

So the basic compound is kvitteringsnummer = receipt number, and kvitteringsnummeret = the receipt number.

This is much more normal in Norwegian than writing separate words.

The ending -et shows that the noun is:

• singular
• definite
• neuter

The head word here is nummer, and nummer is a neuter noun:

• et nummer = a number
• nummeret = the number

In compounds, the last part decides the grammar. So:

• et kvitteringsnummer = a receipt number
• kvitteringsnummeret = the receipt number

Norwegian usually puts definiteness on the noun itself, instead of using a separate word like English the.

So:

• en strekkode = a barcode
• strekkoden = the barcode

That -en ending is the definite form for many masculine nouns.

This is a very common Norwegian use of stå.

When talking about text, numbers, or information on paper or on a screen, Norwegian often uses stå where English uses things like:

• be
• be written
• be printed
• appear
• say

So Kvitteringsnummeret står også under strekkoden means something like:

• The receipt number is also printed below the barcode
• The receipt number also appears under the barcode

You will see this a lot in Norwegian:

• Det står på kvitteringen.
• Hva står det der?

That is the normal placement in a main clause when the subject comes first.

The structure is:

• Kvitteringsnummeret = subject
• står = finite verb
• også = adverb
• under strekkoden = place expression

So står også is the natural, neutral order here.

If you move også, the emphasis can change. For example:

• Også kvitteringsnummeret står under strekkoden would emphasize that the receipt number too is there.

But in your sentence, står også is the most ordinary placement.

Literally, under means under. But in this kind of context, English often prefers below.

So:

• under strekkoden = under the barcode
• very natural English translation: below the barcode

Norwegian uses under very naturally for things printed lower down on a page, label, or screen.

Here, så means so or therefore.

It connects the two ideas:

• the receipt number is also under the barcode
• therefore, the speaker takes a picture of it

So this så expresses a result or consequence.

Both are possible, but ta bilde av is a very common Norwegian expression meaning to take a picture of.

So:

• ta bilde av noe/noen = take a picture of something/someone

Norwegian often leaves out the article in this expression:

• Jeg tar bilde av det = I take a picture of it

You can also say:

• Jeg tar et bilde av det

That version can sound a little more explicitly like I take one picture of it, while ta bilde is very common as a general expression.

Because the fixed expression is ta bilde av noe/noen.

The preposition av works like of in English:

• et bilde av huset = a picture of the house
• jeg tar bilde av det = I take a picture of it

So after ta bilde, Norwegian normally uses av before the thing or person being photographed.

It most naturally refers to kvitteringsnummeret.

So the idea is:

• the receipt number is also under the barcode
• so I take a picture of it

The pronoun is det because nummer is a neuter noun:

• et nummer
• nummeret
• det

In this sentence, så is acting as a conjunction meaning so/therefore. After a conjunction, Norwegian normally keeps ordinary clause order:

• så jeg tar bilde av det

That means:

• conjunction så
• subject jeg
• verb tar

If så were being used more like a fronted adverb meaning then, you could get:

• Så tar jeg bilde av det

So the version in your sentence is normal because så is linking two clauses as so/therefore.