Hi Pocholo.Did Ofnuts' comments help?
If not - see for example:
(define (script-fu-new-layer-white size)
(let*(
(image (car (gimp-image-new size size GRAY)))
(test (car (gimp-layer-new image size size GRAY-IMAGE "New" 100 LAYER-MODE-NORMAL)))
)
(gimp-image-insert-layer image test 0 0)
(gimp-drawable-fill test FILL-WHITE)
(gimp-display-new image)
)
)
(script-fu-register
"script-fu-new-layer-white"
"<Image>/File/Create/New layer..."
"Generates a New layer"
"Pocholo"
"Pocholo"
"Feb 2020"
""
SF-ADJUSTMENT "Layer Size (px)" '(400 1 1200 1 10 0 SF-SLIDER)
And three more comments:
1. If you enter the selection parameter (SF-ADJUSTMENT) must be defined.
2. SF-ADJUSTMENT "Layer Size (px)" '(400 1200 1 10 0 SF-SLIDER) requires 7 arguments (not 6)
3. Location on the menu is probably:
"<Image>/File/Create/New layer..." (not Files)
Python basics are probably easier to learn (and only this has a future...)
Good luck in Your studies