If an employee on a Performance Improvement Plan is not meeting the requirements of the plan, assuming you are meeting your requirements, then termination is the likely outcome. However, this should be a part of your termination policy. You should also have regular communication as part of the PIP, establishing the outcomes if performance doesn’t improve at the start, and have a regular cadence of status/progress meetings.
A PIP only works if both parties want the outcome to be a better working relationship, if your employee doesn’t, or isn’t capable of that, then they are not a good fit for the organization.