First, check if the Data Plugin is shown in the list of Data Plugins installed in DIAdem >> Settings >> Extensions >> Data Plugins.
If the Data Plugin is not there, reinstall the Data Plugin.
If the Data Plugin is there or the issue persists:
- A reinstallation or repair of DIAdem could fix the error.
- Otherwise, uninstall DIAdem and the Data Plugin, then install DIAdem with an offline installer and then the plugin, that could fix a possible error with the installation.