update permission of Frontend folder

 update permision of Frontaend folder 

site > right click you site > Edit Permissiions > select site name from list > edit 

or if still happens the error check web.config

 <features>

                <feature name="EditWebsiteDomains" enabled="true" />

                <feature name="DebugErpRequest" enabled="true" />

                <feature name="ManageAddons" enabled="true" />

                <feature name="UploadAddons" enabled="false" />

                <feature name="ManageDesignPacks" enabled="false" />

                <feature name="UploadDesignPacks" enabled="false" />

                <feature name="ManageLanguagePacks" enabled="true" />

                <feature name="UploadLanguagePacks" enabled="true" />

                <feature name="ManageAdminLanguagePacks" enabled="true" />

            </features>

        </sanaCommerce>

sometime the pipeline reset the permission of the Frontend folder which leads to this kind if issues.