Enforce 15-character minimum password length on Windows

Yes, you can but I don't believe it's supported. You would have to modify Active Directory using ADSIEdit.msc. If you view the attributes for the AD Schema, there is a Min-Pwd-Length attribute. Change this attribute's value to 15 and the minimum password length of 15 will be enforced.

Read more about it here: https://msdn.microsoft.com/en-us/library/ms677113%28v=vs.85%29.aspx?f=255&MSPPError=-2147217396

You can create a registry-based policy that will stuff 15 as the value for minimum password length. A method is described here.

You can do this on a stand alone server/workgroup computer:

1: Open regedit

2: Navigate to "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Network" in the left-hand pane then right-click on the entry.

3: Create a new REG_BINARY value. Name the value "MinPwdLen" and set the minimum password length to seven.

4: Close Registry Editor then restart the computer

You create a policy that applies to all the machines in the domain (a domain-wide policy, make sure it won't conflict with existing policies that set minimum password length by policy means, as you have tried) that will set the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Network\MinPwdLen value to BINARY 0x0F.