Did you even know there was a Psalm 151? The truth is there are many more psalms than the 150 found in the Bible. A psalm is a genre and there is no reason why we can’t write more psalms.
There is a Psalm 151 that is a part of the apocrypha. It is actually two psalms, Psalm 151A (vv. 1-5) and Psalm 151B (vv. 6-7). While Psalm 151 has been a part of the Septuagint, a Hebrew version was found among the Dead Sea Scrolls.
Psalm 151 is a narrative of the choosing of David among his brothers and the killing of Goliath.
You can find the text for Psalm 151 online but I’m using the one found in the The New Oxford Annotated Bible.