disk needs repair, disk utility won't repair

IMAC PowerPC G4; 10.4.11

Using Disk Utility, repaired disk permissions - ran ok.

Then tried to check HD using Disk Utility. Ran verify, got:

"Verifying volume “Macintosh HD”
Checking HFS Plus volume.
Checking Extents Overflow file.
Checking Catalog file.
Checking multi-linked files.
Checking Catalog hierarchy.
%)
Checking Extended Attributes file.
Checking volume bitmap.
Volume Bit Map needs minor repair
Checking volume information.
Invalid volume free block count
Invalid volume free block count
(It should be 4090810 instead of 4090809)",2)

The volume Macintosh HD needs to be repaired.

Error: The underlying task reported failure on exit

1 HFS volume checked
Volume needs repair"

But "repair disk" on Disk Utility is grayed out.

What next?