For Microsoft Certification exams, the legal name on the Microsoft Certification/Learn profile must exactly match the name on the government-issued ID that is presented on exam day. The documentation does not define a special rule for cases where the ID has no last name; it only states that the name must match exactly and that a valid government ID is required.
Because of this, do not guess or invent a last name. Instead, use official support so the profile can be aligned correctly with the ID before scheduling or taking an exam. The appropriate next step is to contact Microsoft Credentials Support and request help configuring the certification profile/legal name to match the ID format (single name, no last name). They handle profile and name issues for exams and can advise what to enter so that Pearson VUE will accept the ID.
If the legal name is already set incorrectly and is blocking an exam, this also needs to be corrected via Microsoft Credentials Support before attempting to test.
References: