When variables are used from Variable Groups, it is not easy to use template expressions or even macro expressions.

You can try the bellow:

1. Define a job variable and escape characters \ and "
2. Use the variable in your task

-job:MyJob
 variables:
    myEscapedSecret: $[replace(replace(variables['STORAGEACCOUNTKEYDBASEBK'], '\', '\\'), '"', '\"')]

 steps:
   -task: xxxxxx
     --storage-key "$(myEscapedSecret)"