The closer the mismatch is to the PAM it's effect on the efficiency will be more severe. I'm not sure if base editors have additional restrictions since you need to make sure the active site is positioned correctly in order to modify the right nucleotide. There are Cas9 variants, as well as other Cas proteins that have different PAM preferences so you may check if one of them will give you a better choice of guide RNA.

Thank you Yoav,

I checked all possible gRNAs, regardless of the PAM, and the one I am using is actually the most suitable one.

Unfortunately, my five T are at positions 1-5 after the PAM. However, I am going to substitute them with other nucleotides subsequently and hope that I get some activity on my targeted mutation.