TextJoin formula with MID function

I tried to run this formula in excel

=TEXTJOIN(",",TRUE,IF(MID($EP3:$ET3,2,255<=6),LEFT($EP3:ET3,1),""))

but it returns #Value! error. Here in the range EP3:ET3, I have values like (L1,L2,R1,R22,R30) etc. basically alphanumeric values.
and according to the digital values I try to group letters together with textjoin which I couldn't make it work.

Can you help me with the formula what is wrong with it, or suggest me an alternative, please? Thanks.