Nusach Sefard says after counting the Omer the Tefillah of “Ribono Shel Olam.”
The Chida in Avodas Hakodesh has an additional special Tefillah for the ending of every week of the Sefira.
The Chok Yakov ( O. Ch.489) writes not to say the kabalistic tefilos after counting the Omer.
“We should refrain from saying the Tefilla because we have no clue of the meaning. These tefilos are only for practicing kabalists who know and understand what they are saying.
Even the Kabalist should also only say it betzina and quietly. “
The Teshuvas Rashbash (189) writes that we have no clue what sefiros are.
The Mishna in Avos says:
"Hashem created the world Basara Ma’amoros". The Kabbalist refer to them as the ten Sefiros , the tools with which Hashem created and runs (sends his Ha’ashpa’a ) his world.
When we say “Ma Shepagamti Bisefirah” (what I have blemished with the sefira.)
The simple explanation is, by doing an Aveirah we somehow block the Tzinor (pipe) from where Hashem’s Ha’ashpa’a flows through to this world.