Hello,
The variable is inaccessible due two reasons:
1) Is not intialized when you first retrieve its value. The variable does not get any value until is started (printed).
I believe FR designer does not let you change any variable value.

So you should set its value at least once by code.
BTW, that is what we do with Xailer:
::DeleteCategory('Xailer')
::AddCategory('Xailer')
::AddVariable('Application:cCompany', Application:cCompany )
::AddVariable('Application:cCopyright', Application:cCopyright )
::AddVariable('Application:cDescription', Application:cDescription )
::AddVariable('Application:cDirectory', Application:cDirectory )
::AddVariable('Application:cFilename', Application:cFilename )
::AddVariable('Application:cTitle', Application:cTitle )
::AddVariable('Application:cVersion', Application:cVersion )
Maybe you can do something similar to that.
2) FR uses the underscore to distinguish between categories and variables. So, do not put them.
This code worked on my side:
oRep:OnBtnGenPdf := {|o| MsgInfo(:GetVariable( "Test" )), .t. }
Regards,