If you can offer salah while sitting on the ground in any way then it is not lawful for you to offer salah on the chair. As the person who is offering salah on the chair does sajdah with gesture, which is not equivalent to true sajdah. And if you are not able to do sajdah in any way then you may offer salah on a chair or ground in sitting position with gesture. However, it is better to offer salah on the ground in both the cases as one is nearer to the ground in this case.
وإن تعذرا ليس تعذرهما شرطاً بل تعذر السجود كاف لاالقيام أومأ … قاعداً وهو أفضل من الإيماء قائما لقربه من الأرض الخ (در مختار مع الشامى 2:568 مطبوعه مكتبه زكريا ديوبند)
Allah knows Best!
Darul Uloom Deoband