From the get-help about_remote_troubleshooting that was referenced in your error message:
HOW TO CONNECT REMOTELY FROM A WORKGROUP-BASED COMPUTER
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ERROR: The WinRM client cannot process the request. If the
authentication scheme is different from Kerberos, or if the client
computer is not joined to a domain, then HTTPS transport must be used
or the destination machine must be added to the TrustedHosts
configuration setting.
When the local computer is not in a domain, the following procedure is required
for remoting.
1. Configure the computer for HTTPS transport or add the names of the
remote computers to the TrustedHosts list on the local computer.
For instructions, see "How to Add a Computer to the TrustedHosts
List" below.
2. Verify that a password is set on the workgroup-based computer. If a
password is not set or the password value is empty, you cannot run
remote commands.
To set password for your user account, use User Accounts in Control
Panel.
3. Use the Credential parameter in all remote commands.
This is required even when you are submitting the credentials
of the current user.
HOW TO ADD A COMPUTER TO THE TRUSTED HOSTS LIST
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The TrustedHosts item can contain a comma-separated list of computer
names, IP addresses, and fully-qualified domain names. Wildcards
are permitted.
To view or change the trusted host list, use the WSMan: drive. The
TrustedHost item is in the WSMan:\localhost\Client node.
Only members of the Administrators group on the computer have permission
to change the list of trusted hosts on the computer.
Caution: The value that you set for the TrustedHosts item affects all
users of the computer.
To view the list of trusted hosts, use the following command:
get-item wsman:\localhost\Client\TrustedHosts
You can also use the Set-Location cmdlet (alias = cd) to navigate
though the WSMan: drive to the location.
For example: "cd WSMan:\localhost\Client; dir".
To add all computers to the list of trusted hosts, use the following
command, which places a value of * (all) in the ComputerName
set-item wsman:localhost\client\trustedhosts -value *
You can also use a wildcard character (*) to add all computers in a
particular domain to the list of trusted hosts. For example, the following
command adds all of the computers in the Fabrikam domain to the list of
trusted hosts.
set-item wsman:localhost\client\trustedhosts *.fabrikam.com
To add the names of particular computers to the list of trusted hosts, use
the following command format:
set-item wsman:\localhost\Client\TrustedHosts -value <ComputerName>[,<ComputerName>]
where each value <ComputerName> must have the following format:
<Computer>.<Domain>.<Company>.<top-level-domain>
For example:
set-item wsman:\localhost\Client\TrustedHosts -value Server01.Domain01.Fabrikam.com
To add a computer name to an existing list of trusted hosts, first save
the current value in a variable, and then set the value to a
comma-separated list that includes the current and new values.
For example, to add the Server01 computer to an existing list of trusted
hosts, use the following command
$curValue = (get-item wsman:\localhost\Client\TrustedHosts).value
set-item wsman:\localhost\Client\TrustedHosts -value "$curValue, Server01.Domain01.Fabrikam.com"
To add the IP addresses of particular computers to the list of trusted hosts,
use the following command format:
set-item wsman:\localhost\Client\TrustedHosts -value <IP Address>
For example:
set-item wsman:\localhost\Client\TrustedHosts -value 172.16.0.0
To add a computer to the TrustedHosts list of a remote computer, use the
Connect-WSMan cmdlet to add a node for the remote computer to the WSMan: drive
on the local computer. Then use a Set-Item command to add the computer.
For more information about the Connect-WSMan cmdlet, see Connect-WSMan.