Thickness tool not removing faces

[JussyAD]

good day all

been trying top remove some front faces to give this shape thickness but to no avail. All I get is an error.
Also, if I try to remove the top, just out of interest, either it doesn't work or it messes up the geometry with some new triangular overlapping face.

if anyone knows what's cooking here please let me know..?

OS: Windows 8.1 Version 6.3 (Build 9600)
Word size of FreeCAD: 64-bit
Version: 0.20.24720 (Git)
Build type: Release
Branch: master
Hash: 8b0c58d1b45017ad42bf0fa8f827f1cf22a398e7
Python version: 3.8.6+
Qt version: 5.15.2
Coin version: 4.0.1
OCC version: 7.5.0
Locale: English/South Africa (en_ZA)

[Willem]

The Mirrored part must have the refined option changed from False to True and then Thickness works.

[JussyAD]

The Mirrored part must have the refined option changed from False to True and then Thickness works.

Sir, many thanks!!! that worked perfectly