Musescore letting me input 5 beats in a 4/4 measure

Score needed to check what might have gone wrong there

Most likely you accidentally (or deliberately, not knowing what it was for) entered Insert Mode. If so, returning the the usual Step-Time note input mode will address it. Crtl+Delete will then delete any beats you erroneously added while in that mode.

Unlikely, that image doesn't show that little grey + above the upper right of that measure

There's plenty of ways to make note values appear to outlast the actual duration of the measure. I doubt that was intentional, but just to check: Are the half notes actually half notes or are they quarter notes with half note heads? Does the triplet extend over a half note (as it appears) or a quarter note? Attached is an screenshot to make note values appear different than they actually are.

No way of telling without seeing the acttual score

How can make that happen in my sheet i want to add an extra beat to that measure?

Use insert mode. See https://musescore.org/en/handbook/3/note-input#note-input-modes

I was having a similar issue, but it was adding rests in other parts instead of its own.

Looks like your score has probably become corrupt. Please ask for help on the support forum - this issue is about a different problem. When you post to the forum, be sure to attach the actual score, not just a picture of it, and describe how to reproduce the problem. Then we can understand and assist better.