Trying to set up Geany on my home computer and the Spellcheck plugin is refusing to work.
I have installed enchant_1.5.0-2_win32.zip over the Geany package as suggested on the portable version's site and the only change is that the menu entry is no longer greyed out and there is a "Default (en)" option listed there.
We cannot support instructions from other people without knowing why they suggest this. Please either use the library versions bundled with the Geany installer or ask for support at the authors of portable version. Enchant 1.5 most probably won't work on the Windows version of the plugin. At the very least it is not supported and much newer versions are bundled with the Geany installer.
The plugin still doesn't work and interacting with the plugin via the Plugin Manager or plugin Preferences brings up the same error.
_Also, if you click away from this error, you can't alt-tab back to the message box, effectively killing Geany since you can't close the window or interact with it in any way. Though that may be a GTK/Windows issue._
Hard to say what is causing this but it might all be related to the old Enchant version you have installed.
Did you read the instructions on https://plugins.geany.org/spellcheck.html#configuring-custom-dictionaries?