You have to be in the context (double click to enter group) of the face which has the texture and you must only have one face selected. This means you may need to turn on hidden geometry so you can select just one face. If you have 'painted the group/component from the outside you will not get the context menu on the face, which is why you have needed to explode things. You should be adding materials to faces not containers.
That texture menu is only for the specific face you are on and that itself needs to have a texture applied to it.
As Box mentions, it won’t be there if you have applied it to the group and not a specific face - this is why exploding it works as that flattens it and applies it to the face.
If the scale of the texture is incorrect, then sometimes its simply easier to adjust the texture size itself which will change it everywhere it is used.
No, you should understand what a Group or Component is by now, you double click to open the G/C for editing. This gets you into the ‘Context’ of the face you want to work with.
Sorry, been on the road four 8 hours today and can’t quite think how to make this simple for you, perhaps after a nights sleep.
You can see in Entity Info if the sleeted face has a material. If it has no material of its own, SketchUp displays it with the material “inherited” from the parent groups/components. Texture Positioning is only available if the face itself has a material.
A not yet painted face inside a group or component “inherits” displays the container’s
material.
This selected face itself in ‘Entity Info’ shows: Front Default material / Back Default material (if back has not been painted).
A painted face inside a group or component displays its own material, overriding the container’s material.
This selected face itself in ‘Entity Info’ shows: Front material A / Back Default material (or material B if also painted (or somehow painted translucent as happens with translucent materials).
A selected group or component displays its material, or if faces inside are painted these locally override the container’s material.
This selected G/C itself in ‘Entity Info’ shows: Front Default material (or the applied material). There is no Back to show.
Inherit comes into play when exploding a group or component. Default faces and edges inside inherit the container’s material.