I am getting error as this value must be a scalar in polyroots function

Which version and maintenance release of Mathcad are you using?

Maybe the problem is due to a bug in an older version which already is fixed?

Do you get the error already in the last line of your sheet when you just define the function r or when you try to evaluate it via r(10)=

I also experience no problem in evaluating your function r (Mathcad 15, M045).

When calculating your various coefficients a(i), b(i), etc. every call evaluates the solve blocks again and again. This could be speeded up significantly by creating a function which calls the solve blocks just once and then calculates and returns ALL of the necessary coefficients in a vector.

Doing so would be quite some work in rewriting the sheet but probably worth the time spent.

Her a small example - the function I1(i): Your version of this function calls the solve block for f three times while the version below calls it only once (and we don't need the function x1(i) and x2(i) you defined above anymore):

using this new function I1 and a similar one for I2 already cuts the time to calculate r to one third and the whole sheet could be speeded up much, much more, I guess.

Nevertheless you experience that strange error. So lets try some changes (I am just poking around in the dark):

What do you get when you evaluate B(10)=? Do you get

If yes, try to reformulate your function r

Do you get the result or do you still experience that error?