Q:

Does "not to be" mean "does not" or "should not"?

This page says

In statistics, the likelihood function (often simply called the likelihood) expresses the plausibilities of different parameter values for a given sample of data. While not to be interpreted as a probability, it is equal to the joint probability distribution of a random sample. However, whereas the latter is a density function defined on the sample space for a particular choice of parameter values, the likelihood function is defined on the parameter space while the random variable is fixed at the given observations.

What does "While not to be interpreted as a probability" mean here? Especially "not to be".
Does it imply this meaning (my own version)?

although mathematician/statistician does interpret this object as a probability...

A:

Here "not to be" means "ought not to be". It is not appropriate to so interpret it. It would be an error to interpret it is a probability.
The part you are asking about could be rephrased thus:

Yes, it would be wrong to interpret it as a probability, however...

