Ok so Savitar's timeline is very confusing but I think I understand it and am going to explain it below. Just remember that Flashpoint created this whole problem.

Original Post-Flashpoint Timeline:

In the year 2016, Barry encounters Savitar

Wally accidentally frees Savitar from his prison

Barry never discovers Savitar's identity

Savitar kills Iris

Barry creates Time Remenants to save Iris and still fails. The only ones who survive are the Original Barry and 1 Time Remenant

Savitar cripples Wally and brain damages him

Savitar disappears

Team Flash disowns TR Barry because he isn't the original

In the year 2020, TR Barry becomes Savitar and travels the multiverse killing Speedsters

Savitar returns to Earth 1 to kill Barry

Barry traps Savitar in the Speed Force and believes he trapped the current version of Savitar that had already killed Iris

Current Post-Flashpoint Timeline:

In the year 2016, Barry encounters Savitar

Barry is sent to the future and watches Iris die (Now that this alteration has happened, time is in flux and Barry can now make decisions that Savitar never did, these memories get added to Savitar's the moment Barry does it)

Barry tries many different things to change the future so that Iris doesn't die

Savitar tricks Wally into freeing him as he already knows how to do it

Barry travels to the Future and learns how to make the Speed Force Trap

Barry learns Savitar's identity

Team Flash builds the Speed Force cannon

H.R takes Iris's place with the Face changer technology

The cannon doesn't work as Savitar learned how to use the Philosophers stone to stop the Cannon

Savitar kills H.R (Who he thinks is Iris)

Now that Iris has been saved, Savitar's existence becomes a paradox and Team Flash fights him until he is erased from existence

Hopefully that is a good explanation, it is a long one though