The UI of Tag synonyms is a bit poor, so there are always some subtleties to take into account. In this case, the situation seems to be like this:
- expresiones is already a synonym target for two other tags: expresión and fixed-expression, as seen in 'expresiones' Tag Synonyms.
- The synonym creation does not allow suggesting a new mapping (A -> B) if the source (A) already has some tags pointing to it. That is, if A->B already exists, you cannot do B->C unless you remove A->B.
For this, the solution would be to:
By the way, some mapping are not necessary: as seen in the Tags list we now have expresiones, expresiones-fijas, expresión, expression, fixed-expression, and fixed-expressions. There is no need to have all the tuples singular-plural, since they share a big part of the initial characters and user interface suggests tags based on the characters you type and match the beginning of the tags' names.
See What are tag synonyms and merged tags? How do they work? in Meta.SE for more info.