@Sonia de Lucas García, Thanks for posting in Q&A.
According to your description, I know that you want to know if you can access MS Purview Compliance Portal with a free account.
Based on my researching, I found that you can access MS Purview Compliance Portal with a free account and you can enter on the new page https://go.microsoft.com/fwlink/p/?linkid=2077149 to enter the Purview Compliance Portal.
Moreover, if you want to access services like Content Search on the MS Purview Compliance Portal with a free account to try to workshop, it is recommended that you assign Organization Manager and eDiscovery Manager permissions to your account.
There are some links about MS Purview Compliance Portal and some Permissions you can refer:
Microsoft Purview compliance portal | Microsoft Learn
Get started with Content search | Microsoft Learn
Assign eDiscovery permissions in the Microsoft Purview compliance portal | Microsoft Learn
Hope above can be helpful.