CHAR columns are fixed length, so they are right padded with blanks. If you concatenate '4535 ' with 'AB' you will produce '4535 AB', which is too long to fit in your column.
To avoid this problem use the RTRIM function to remove trailing blanks. Your update becomes
update nsr_pmt set pmt_id = rtrim(pmt_id) || 'AR';
Please note also that the concatenation operator in Oracle is ||, not +. That would also have generated an error.