Translation:Please sit on a chair.
I agree that 'please take (/have) a seat' should be accepted, because in the West 'on a chair' is generally taken for granted and left unsaid, unless you mean 'and not on the couch,' etc. In fact, it seems to me いすに may be specified in Japanese to avoid having すわってください understood as 'please sit on the floor / tatami. Of course, the kind of room you are in may clarify this anyway. Or you could use a different Japanese verb, that excludes sitting on the floor, こしかけてください (which if literally construed means, hang your hips [on something]), therefore making the specification of a chair basically redundant, as it seems in English, unless we are trying to exclude the sofa, etc.
Actually as far as i know suwaru in referred to sitting down on the floor... the japanese way; there is another way to say sit down our western way i do not remember but something like koshi kakeru... Anyway to me "please sit down" or please take a seat should be accepted but maybe im wrong
No one has mentioned this but in addition to the on/in distinction I wonder if 'the' chair as opposed to the given 'a' chair might be more natural. 'A' chair assumes the room has many chairs everywhere and you should just grab one. If there were an intended chair to sit on/in we'd use 'the.'