follow this way:
from odoo import fields, models

class ProductCategory(models.Model):

    _inherit = 'product.category'

    x_complete_name_length = fields.Integer(string="Complete Name Length", compute='_compute_complete_name_length', store=True, readonly=True)

    def _compute_complete_name_length(self):

        for category in self:

            if category.complete_name:

                category.x_complete_name_length = len(category.complete_name)

            else:

                category.x_complete_name_length = 0

The error you're encountering, "forbidden opcode(s) in 'lambda': STORE_ATTR," is due to the use of the self keyword within the lambda function, which is not allowed in Odoo field computation. Odoo's field computation function doesn't directly accept the self parameter.