I’ve upgraded to Sketchup Go in order to be able to export to .OBJ, as per the information on File import and export capabilities | SketchUp
After having upgraded, the export to .OBJ is still blocked, and a further upgrade is required.
Which export capabilities Sketchup Go actually provides?
OBJ export is available in SketchUp Go.
If it’s not available for you, you probably need to sign out of SketchUp, clear your browser caches, and sign back in.
SketchUp Go is the name of the Subscription - which includes Sketchup for Web - this can export .OBJ
I don’t think SketchUp for i.Pad itself can’t export any other 3D formats beyond Pixar’s USDZ - a favourite of IOS.
Working with SketchUp Files | SketchUp Help
You can open your files created in SketchUp for i.Pad in SketchUp for web and export from there - even on an i.Pad