The error is caused by your data - the column
number_qanema cannot be converted to a decimal value.
Change your query as follows;
SELECT
MAX(CAST(number_qamema AS DECIMAL(15,4))) AS NB_QE
FROM
NB_QE
WHERE
ISNUMERIC(number_qamema) = 1
AND
name_costomer1 = 'waleed'
GROUP BY
name_costomer1
Kind Regards