Some ERP vendors, especially those that offer only cloud versions of their software, also offer free trial versions to enable prospective users to 'kick the tyres' of their solutions. Usually these are full versions of the product with minor limitations in terms of how many users or how much evaluation time are allowed by the trial version.
Obviously vendors hope that some businesses might fast-track their selection by using the trial version and minimize the amount of effort required from the vendor to just a handful of phone calls. This is a pipedream.
Unless you have a very well-thought through and structured approach to using a trial version or you are only evaluating a module of an overall ERP system (e.g. HR), evaluating ERP solutions via the trial version route is probably a waste of time.
For example, while you may be able to evaluate Quickbooks or Xero this way, no business is likely to buy SAP by using a trial version even if this were available.
In my view, for ERP selections, asking vendors to demonstrate against your carefully defined demo scenarios is a much more effective way to go than burning up time 'hunting' around for stuff in a trial version.