I'm trying to develop a module. I made simple changes to the equipment form inside maintenance module. But when I'm trying to install this module I get an error:

TypeError: Model 'maintenance' does not exist in registry.

In the manifest file I have maintenance inside dependencies:

'depends': ['base','web','maintenance'],

This is my models:

# -*- coding: utf-8 -*-
from odoo import models, fields, api
class custom_category_fields(models.Model):    
    _inherit = 'maintenance.equipment'
    CPU1 = fields.Char()    
    GPU1 = fields.Char()

And this is my views:

<record model="ir.ui.view" id="view_equipment_form_inherit" >
     <field name="name">equipment.specs</field>
     <field name="model">maintenance.equipment</field>
     <field name="inherit_id" ref="maintenance.hr_equipment_view_form"/>
     <field name="arch" type="xml">
         <xpath expr="//field[@name='note']" position="after">            
            <group>                
                <field name="CPU1"/>
            </group>                        
            <group>                
                <field name="GPU1"/>
            </group>        
        </xpath>    
    </field>
</record>

What is wrong?