How did you test the power at the main fuse ?
With the Ignition Switch in “On” position, you need to connect the Negative (-) wire on your Multimeter to the Negative (-) terminal of the battery itself.
Then you need to connect the Positive (+) wire on your Multimeter to the one side of the brass fuse terminal to the fuse first, and see if you get a Voltage reading.
If you don’t get a reading - connect the positive (+) wire of the Multimeter to the brass fuse terminal on the other side of the fuse to see if you get a Voltage reading.
Don’t try to get a Voltage reading on the fuse itself - a bad or corroded connection between the fuse and the fuse terminals itself can easily cause some confusion.
If you don’t get a Voltage reading at the brass fuse connections at all - there is a possibility that the wire connections to the brass fuse terminals are corroded or broken.
In my scooter’s case I exerienced a similar problem - the connection between one fuse terminal and the fuse wire was corroded. When I started fiddling with the wire on that side of the fuse the starter on the scooter suddenly started working again.