MailWizz is not related to WordPress.
MailWizz is no WordPress plugin.
MailWizz is a standalone program (EMA = Email Marketing Application), which one usually installs in a separate, dedicated directory and at least on a subdomain, if not better a domain.
Nested installations are not recommended (i.e. it is not recommended to install it in a subdirectory as otherwise there can be interference with program(s) in the parent/sub-directory).