How to add a description on the opportunity through the Sugar CRM app?

Could you specify what do you mean by interaction history you need to add? log Call/Meeting held or add new Note in the context of Account?

I just need to put a note in that field where "description" is written.

I know how to add a interaction history about call/ Meeting ir note...  but none of these options goes into the field where "description" is written.

Try to click three vertical dots in upper right corner. In case menu appeared, select Edit

It sounds like your Mobile Detail vs EditView layouts are different.

Go to Studio -> Opportunities -> Layouts -> Mobile Layouts, and ensure that the Description field is on the "EditView" layout. Save and Deploy, then visit the Opportunity on your mobile app, and as Dmytro said above, use the three vertical dots in upper right corner, and select "Edit".