Rrerku

I would like to create a document used to test diverse packages, and I was wondering if it was possible to use a command so that part of the code is read only if the corresponding package is loaded. I've come up with this but it throws an error which I don't know how to fix.

documentclassarticle
usepackage[utf8]inputenc

makeatletter
newcommandIfPackageLoaded[3]@ifpackageloaded#1#2#3
makeatother

begindocument
IfPackageLoadednatbibnatbib loadednatbib not loaded
enddocument

I get this error:

! LaTeX Error: Can be used only in preamble.

See the LaTeX manual or LaTeX Companion for explanation.
Type H <return> for immediate help.
 ... 

l.10 IfPackageLoadednatbibnatbib loaded
 natbib not loaded
Your command was ignored.
Type I <command> <return> to replace it with another command,
or <return> to continue without it.

I also know that the command ltx@ifpackageloaded exists when loading the package ltxcmds but when I try to use it in

documentclassarticle
usepackage[utf8]inputenc
usepackageltxcmds

begindocument
ltx@ifpackageloadednatbibnatbib loadednatbib not loaded
enddocument

I get this error message:

! Undefined control sequence.
l.11 ltx
 @ifpackageloadednatbibnatbib loadednatbib not loaded
The control sequence at the end of the top line
of your error message was never def'ed. If you have
misspelled it (e.g., `hobx'), type `I' and the correct
spelling (e.g., `Ihbox'). Otherwise just continue,
and I'll forget about whatever was undefined.

Can someone please point out what I did wrong?

EDIT: changed documentclass from minimal to article.

As users Ulrike Fischer and moewe pointed out, this works:

documentclassarticle
usepackage[utf8]inputenc
usepackageltxcmds

makeatletter
newcommandIfPackageLoaded[3]ltx@ifpackageloaded#1#2#3
makeatother

begindocument
IfPackageLoadednatbibnatbib loadednatbib not loaded
enddocument