I caught chansey but when i close the menu appears a missigno and sometimes a venussaur. Thx in advance

You're probably not buffering some of the inputs correctly during the manip, as I don't believe either of those is an IGT fail. If you are getting both Venusaur and Missingno. from the same save, this is almost certainly the case. The Chansey you're supposed to get has a specific special stat to get an Alakazam

(Also join the discord if you haven't already, people tend to check that more frequently)

Accidentally reproduced the Missingno. during some practice today, it's caused by pausing movement during the left-right at the end of the movement. Venusaur is probably a similar issue.