I had a similar issue and, asking in PowerShell's discord server, the answer turned out to be that there wasn't any documentation available for my culture. Doing
Update-Help -UICulture en-US -Force
solved the problem by downloading the documentation for the en-US culture.
However, as mklement0 mentioned in his answer, the help for this particular module seems to be broken. Looking into PowerShellGet files, the module uses different names for the module (PowerShellGet), and the file containing the code (PSModule.psm1). Although, this shouldn't be a problem, as the module uses the #.ExternalHelp directive for function help which doesn't have any specific naming requirements, it seems it doesn't work correctly.
A quick fix for this that apparently works is to change the name of the help file from 'PSModule-help.xml' to 'PowerShellGet-help.xml'. This file is under
$HOME\Documents\PowerShell\Help\PowerShellGet\en-US
for Windows systems, or
$HOME/.local/share/powershell/Help/PowerShellGet/en-US
for non-Windows systems.
Note, that even if you do this, you won't be able to get help for Get-CredsFromCredentialProvider as it is missing from the .xml help file. However, this is an internal function that wasn't meant to be exported in the first place, so the missing documentation is actually deliberate. Still, if you want to check out what it was, you can check it out here (though there's not any interesting content whatsoever :) )!