My metrics are:
cap height 777
The red line indicates that the layers in the glyph are not compatible and therefore cannot be interpolated.
If you do not intend to interpolate, you can add the Enforce Compatibility Check parameter and turn it off, like this:
it works! thank you!
But I am still out of curiosity wondering why they would not be compatible, all three masters have the same metrics
Could be differences in the number of paths, nodes, anchors, order of shapes, and such. Take a look at the Multiple Masters, Part 2: Keeping Your Outlines Compatible tutorial and perhaps the others in that series.