Hi jk_...,
this just means your MSA has the wrong input format. Please check the manual for guidelines for the supported formats by classic RAxML.
The standard extended phylip format used by RAxML looks like this
4 10
taxon1 GGGGGACGTG
taxon2 AGAGSACGTC
taxon3 GGAGTACCTC
First number first row is the number of tips/OTUs/accessions, second number the number of characters (nucleotide, amino acid or multistate: 0,1,2,...)
"Extended" means the length of the taxon name can be longer than 10digits, which was the limit of classic PHYLIP.
Cheers /G