I got that error how i can fix it.

2019-12-20 05:37:45,170 7509 ERROR odoo15 odoo.modules.registry: Failed to load registry
Traceback (most recent call last):
File "/odoo13/odoo-server/odoo/modules/registry.py", line 59, in __new__
return cls.registries[db_name]
File "/odoo13/odoo-server/odoo/tools/func.py", line 69, in wrapper
return func(self, *args, **kwargs)
File "/odoo13/odoo-server/odoo/tools/lru.py", line 44, in __getitem__
a = self.d[obj].me
KeyError: 'odoo15'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/odoo13/odoo-server/odoo/modules/registry.py", line 85, in new
odoo.modules.load_modules(registry._db, force_demo, status, update_module)
File "/odoo13/odoo-server/odoo/modules/loading.py", line 416, in load_modules
force, status, report, loaded_modules, update_module, models_to_check)
File "/odoo13/odoo-server/odoo/modules/loading.py", line 312, in load_marked_modules
perform_checks=perform_checks, models_to_check=models_to_check
File "/odoo13/odoo-server/odoo/modules/loading.py", line 179, in load_module_graph
load_openerp_module(package.name)
File "/odoo13/odoo-server/odoo/modules/module.py", line 365, in load_openerp_module
__import__('odoo.addons.' + module_name)
File "/odoo13/odoo-server/custom/pos_multi_session/__init__.py", line 2, in <module>
from . import models
File "/odoo13/odoo-server/custom/pos_multi_session/models/__init__.py", line 2, in <module>
from . import pos_multi_session_models
File "/odoo13/odoo-server/custom/pos_multi_session/models/pos_multi_session_models.py", line 15, in <module>
class PosConfig(models.Model):
File "/odoo13/odoo-server/custom/pos_multi_session/models/pos_multi_session_models.py", line 49, in PosConfig
@api.multi
AttributeError: module 'odoo.api' has no attribute 'multi'
2019-12-20 05:37:45,295 7509 INFO odoo15 werkzeug: 127.0.0.1 - - [20/Dec/2019 05:37:45] "GET /web/database/selector HTTP/1.1" 200 - 13 0.014 0.297
2019-12-20 05:37:45,322 7509 INFO ? werkzeug: 127.0.0.1 - - [20/Dec/2019 05:37:45] "GET /web/static/lib/fontawesome/css/font-awesome.css HTTP/1.1" 304 - - - -
2019-12-20 05:37:45,322 7509 INFO ? werkzeug: 127.0.0.1 - - [20/Dec/2019 05:37:45] "GET /web/static/lib/bootstrap/css/bootstrap.css HTTP/1.1" 304 - - - -
2019-12-20 05:37:45,328 7509 INFO ? werkzeug: 127.0.0.1 - - [20/Dec/2019 05:37:45] "GET /web/static/lib/jquery/jquery.js HTTP/1.1" 304 - - - -
2019-12-20 05:37:45,329 7509 INFO ? werkzeug: 127.0.0.1 - - [20/Dec/2019 05:37:45] "GET /web/static/lib/popper/popper.js HTTP/1.1" 304 - - - -
2019-12-20 05:37:45,334 7509 INFO ? werkzeug: 127.0.0.1 - - [20/Dec/2019 05:37:45] "GET /web/static/lib/bootstrap/js/carousel.js HTTP/1.1" 304 - - - -
2019-12-20 05:37:45,340 7509 INFO ? werkzeug: 127.0.0.1 - - [20/Dec/2019 05:37:45] "GET /web/static/lib/bootstrap/js/modal.js HTTP/1.1" 304 - - - -
2019-12-20 05:37:45,341 7509 INFO ? werkzeug: 127.0.0.1 - - [20/Dec/2019 05:37:45] "GET /web/static/lib/bootstrap/js/dropdown.js HTTP/1.1" 304 - - - -
2019-12-20 05:37:45,346 7509 INFO ? werkzeug: 127.0.0.1 - - [20/Dec/2019 05:37:45] "GET /web/static/lib/bootstrap/js/tooltip.js HTTP/1.1" 304 - - - -