Are we not allowed to place fillblock over a glass layer? I get a DRC error saying "can't overlap those layers" when I do this

Both types are defined on the same plane in magic, which makes them automatically flagged if they are occupying the same area. If you really need fillblock under glass cut, and you've put them in separate cells to make that happen (which you presumably did), then it will work, and you can ignore the error.

Does that mean that it would pass precheck and tapeout with that error? (if glass cut and fillblock4 are in separate cells)

My project passes precheck and tapeout despite the DRC error showing up in magic.

Magic's DRC checks are "advisory" for the tapeout process, not "mandatory", so if you know you can ignore a specific error, it won't block the tapeout.

@Tim Edwards I think I may be adding the fillblock wrong because when I check my GDS after tapeout, there is fill within my fillblock area wherever I have glass cut.

That's partly my fault for putting glass cut and fill block on the same plane in magic so they can't both be present at the same time. I wasn't anticipating user-generated glass cut areas. The fill generation script flattens the layout, so having the fill block or the glass cut in a separate cell doesn't help---they get merged into the same plane during that flattening step. I will probably need to make sure that fill block gets defined on a different plane then glass cut, and I will have to make sure that change gets updated on the Efabless servers where the fill generation script is being run.