Units not compatible

Its absolutely unclear to me what you are trying to achieve!

What are the units of the values in P?

Have you read the help concerning the "linterp" function?
This function returns a function, not a value.

And of course the first argument (a column vector) must be of the same dimension (any temperature unit in your case) as the third argument (independent variable of the function).

So this would work OK

but I doubt that your intention is to interpolate between two equal values (2).