Note:
This answer shows how to escape / quote the @ char. properly in the context of PowerShell's usual parsing rules.
If your command line only contains verbatim arguments - i.e., only literal tokens, not PowerShell variable references (e.g, $file) or expressions (e.g., ($dir + '\script.ps1')) - you can alternatively place --%, the stop-parsing token, before the pass-through arguments, as shown in programmer365's answer; note that cmd.exe-style variable references such as %FOO% are still expanded, however, and that the stop-parsing token has many limitations and pitfalls - see this answer.
@ is a metacharacter in PowerShell (a character with syntactic meaning[1]), so in order to pass it verbatim through to az you must either quote the whole argument or `-escape the @ individually:
With a literal script filename:
# Either: `-escape the @
az ... --scripts `@script.ps1
#`# Or: quote the whole argument
# Use '...' for a literal argument.
az ... --scripts '@script.ps1'
With the script filename stored in a variable, $file:
# Either: `-escape the @
az ... --scripts `@$file
#`# Or: quote the whole argument
# Use "..." for an argument with variable references, i.e. an expandable string
az ... --scripts "@$file"
Note: You could get away with just @$file in the variable case, but given that that doesn't work with any char. other than $ following the @, it's better to get into the habit of always quoting / escaping a verbatim @.
[1] @ has several syntactic uses, and the specific use depends on what character comes next. In your case, the @ in @script.ps1 was interpreted as the splatting operator with a variable named script, with the .ps1 part interpreted as an attempt to access a property named ps1 on that variable - hence the error message.
