Can an executor of a will also be a beneficiary?

There is no conflict of interest, even if there is just one executor and one beneficiary and they are the same person.

It is common, for example, for a spouse or civil partner to leave everything to the survivor and to appoint the survivor as sole executor.

Witnessing a will

A beneficiary should not act as a witness to a will. Doing so does not invalidate the will, but it will almost certainly invalidate any gifts to the witness.