Problems in applying fillets in solidworks

Cannot apply fillets of 1mm in the highlighted area.But fillets more than 1mm can be applied

2 Answer

Though I can't see precisely why it won't work with 1mm in this example (it starts working at 1.8mm), I found a workaround utilizing 'split' feature to isolate the geometry needing filleted and 'combine' feature to put it back together again.

A 1mm fillet works fine for me.
If you had to, you could reorder the feature tree to place the cut and fillet before the boss-extrude2 feature.