When See Electrical starts up there are 3 main folders which contain important information. The Font.dat files are found in the 'Templates' folder so this error indicates that the folder and the files within it cannot be found.
This is often due either to the 'File > System Settings > Folder Locations' not being set correctly within See Electrical, or more commonly because the templates file is on a network and the connection has not been established yet.
If when you open Windows File Explorer and a mapped drive is displayed with a red cross
through it you may need to click on that mapping to establish the connection and remove the error.