Hi Jurgen,
Thanks for your answer and the advises. I try to mantain alwaysthe descriptions, I've configured this in spanish, but for the screenshots I've enter in the system in english, may be I've missed some translations. I'll try to take in account this always.
I've deleted the groups only when I've tried several things and it wasn't working fine. But you are right I've deleted the groups before to delete the corresponding strategies. I thought deleting the groups would also delete the dependant entries in other tables, but not. I think I made a mess in the tables... 
I see the table T16FS and there is a lot of entries, more than the corresponding of the groups I've deleted. I know some entries could be of PR or other documents, but some of them are from PO that I didnt delete. This is an IDES system, so may be this is the problem, other people may have changed these things and now it's a mess...
I've deleted the entries I think that were from PO release strategies from several tables T16FK, T16FS, T16FC... (I know I shouldn't delete the entries directly from the table, its not a good practice and it could be dangerous by the relationships of the tables, but I didnt know other way to fix it)
Now, if I check the PO strategies seems that is ok,
But its still not working 
If I create a PO I cannot see the release tab. If I go to ME29N appears the typical message "Purchasing document 4XXXXX cannot be released"
I'm going to try to debug this or check other tables. I'm afraid to delete more tables entries, I dont want to make bigger this mess.
Any advice would be greatly appreciated.
Thanks.