Question: Error with binomial

Hi,I want to compute some expressions as binomial(d-2,r)*binomial(v-d-1,r-1) and something like that. In the process, there are many such expressions but with differernt parameters. Some of them can pass successfully, but some of them will generate a mistake. That is: Error, (in gcd/LinZip) variable in input expression must be in either the evaluate or retained variables.Could you please what happened to it? Thanks.
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