I have installed Ultimate Belts Space Age, but it doesn't give me the option to craft the loaders associated with that mod—though it does allow me to build them for other installed mods.
Sorry, English is not my native language.

I've allowed this to work with https://mods.factorio.com/mod/UltimateBeltsSpaceAge_fix21

Yes, I am using the mod
Ultimate Belts Space Age - Factorio 2.1 Fix
The other mods I am using are:
Base
Infinity-invo
Elevater-rails
Flib
FNEI
Infinity-item-recipe
InsertersDontNeedPower2_x
Item-network-sa-continued
Krastorio2Assets
kry_stdlib
longer-pipe-to-ground
manual-inventory-sort
recycler
sienceception
themightygugi_longreach_forked
UltimateBeltsSpageAge_fix21
valves.lib
bottleneckLite
ChangeInserterDropLine
quality
valves
space-age
BuggisBots_Fork_Moe
Infinity_Inserter_Capacity_Bonus_Research
Infinity_worker_Robot_capacity_Reseatch
Krastorio2MenuSimulations
loaders-modernized
some-gleba-science
Tintrifuge
Krastorio2
kry-inserter
loaders-modernized-integrations
science-spread
Kux-Corelib
Kux-speedbuttons

I know it’s a long list and quite a few mods, but they are the same ones I was using in version 2.0, or their updated versions for 2.1.
I hope you can help me identify where the problem lies; thanks.