Please see [Choosing an appropriate standard deviation for prior) and if this does not answer your question, report back. Formal prior elicitation is done in Del Negro Schorfheide (2008) - Forming priors for DGSE models.
Regarding the error message: you are using a beta distribution that implies the parameter is between 0 and 1. Given that no realizations can be bigger than 1 or smaller than 0, this limits the standard deviation that is possible. If you impose a mean of 0.86, the parameter can at most deviate from the mean by 0.14 in the right tail. You could increase the standard deviation by putting more mass in the left tail where deviations of up to 0.86 are possible, but this would at some point shift the mean. This is the intuition why your combination of mean and variance is unfeasible.