If have already caught all of the tapus and you release one of them are you able to go back a try to catch another tapu.
You are only allowed to get the island guardians once, sorry.
Source: this question, plus personal experience (I learned the hard way, oops)
No, you aren't able to catch a legendary ever again on that save file if you release it. But if you mean release 1 tapu and catch a different tapu then yes you are able to do that.
Hope I helped!