The person operating a piece of software
Solution 1:
We have two words for this.
Operator
1: one that operates: such as
a: one that operates a machine or deviceoperator - merriam-webster.com
This is the formal of the two. Often used in government documents.
End-User
the ultimate consumer of a finished product
end user - merriam-webster.com
This is more often used in documents produced by private software development companies.
In product development, an end user (sometimes end-user) is a person who ultimately uses or is intended to ultimately use a product. The end user stands in contrast to users who support or maintain the product, such as sysops, system administrators, database administrators, information technology experts, software professionals and computer technicians.
end user - wikipedia.org
Neither word is exclusive to software.
There are many roles to play when working with software. Users who play those roles are often named after the role; e.g., user who administrates: administrator.