SQL Server proxy (user) not a valid user error 14529 (Windows 10 Pro)

Trying to replicate a server database (SQL Server Standard 2017) to a laptop (SQL Server Express 2017), following the steps outlined by Microsoft in the online tutorial " Tutorial: Configure replication between a server and mobile clients (merge)."

The PCs are connected on a workgroup, no domain. The problem arises when I try to create the merge subscription on the server PC. As instructed, I use the account <Subscriber_ Machine_Name>\repl_merge

When I try to accept the creation of this new subscription, I get an error telling me that the sql server proxy <Subscriber_ Machine_Name>\repl_merge is not a valid user 14529

The user <Subscriber_ Machine_Name>\repl_merge is a Windows user on the laptop, and is a security login to the installation of SQL Server Express.

Any assistance with this error greatly appreciated.