After installing the newest version getting errors

no your error is an old depreciate code "isadmin" but i didn't think it flexicontent
please set error_reporting to maxium and debug to yes in joomla configuration
you will see more error information

This is what the debug says:
Call stack
# Function Location
1 () JROOT/plugins/editors-xtd/upagebutton/upagebutton.php:17
2 PlgButtonUpagebutton->__construct() JROOT/libraries/src/Extension/ExtensionManagerTrait.php:234
3 Joomla\CMS\Application\CMSApplication->loadPluginFromFilesystem() JROOT/libraries/src/Extension/ExtensionManagerTrait.php:157
4 Joomla\CMS\Application\CMSApplication->loadExtension() JROOT/libraries/src/Extension/ExtensionManagerTrait.php:96
5 Joomla\CMS\Application\CMSApplication->bootPlugin() JROOT/libraries/src/Plugin/PluginHelper.php:221
6 Joomla\CMS\Plugin\PluginHelper::import() JROOT/libraries/src/Plugin/PluginHelper.php:181
7 Joomla\CMS\Plugin\PluginHelper::importPlugin() JROOT/libraries/src/Editor/Editor.php:309
8 Joomla\CMS\Editor\Editor->_loadEditor() JROOT/libraries/src/Editor/Editor.php:170
9 Joomla\CMS\Editor\Editor->display() JROOT/plugins/flexicontent_fields/textarea/tmpl/field_default.php:45
10 include() JROOT/plugins/flexicontent_fields/textarea/textarea.php:532
11 plgFlexicontent_fieldsTextarea->onDisplayField() JROOT/components/com_flexicontent/classes/helpers/flexiutilities.php:428
12 FLEXIUtilities::call_FC_Field_Func() JROOT/components/com_flexicontent/classes/flexicontent.fields.php:505
13 FlexicontentFields::getFieldFormDisplay() JROOT/administrator/components/com_flexicontent/views/item/view.html.php:693
14 FlexicontentViewItem->_displayForm() JROOT/administrator/components/com_flexicontent/views/item/view.html.php:49
15 FlexicontentViewItem->display() JROOT/administrator/components/com_flexicontent/controllers/items.php:1470
16 FlexicontentControllerItems->edit() JROOT/libraries/src/MVC/Controller/BaseController.php:672
17 Joomla\CMS\MVC\Controller\BaseController->execute() JROOT/administrator/components/com_flexicontent/admin.flexicontent.php:264
18 require_once() JROOT/administrator/components/com_flexicontent/flexicontent.php:22
19 require_once() JROOT/libraries/src/Dispatcher/LegacyComponentDispatcher.php:71
20 Joomla\CMS\Dispatcher\LegacyComponentDispatcher::Joomla\CMS\Dispatcher\{closure}() JROOT/libraries/src/Dispatcher/LegacyComponentDispatcher.php:73
21 Joomla\CMS\Dispatcher\LegacyComponentDispatcher->dispatch() JROOT/libraries/src/Component/ComponentHelper.php:355
22 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/AdministratorApplication.php:143
23 Joomla\CMS\Application\AdministratorApplication->dispatch() JROOT/libraries/src/Application/AdministratorApplication.php:186
24 Joomla\CMS\Application\AdministratorApplication->doExecute() JROOT/libraries/src/Application/CMSApplication.php:294
25 Joomla\CMS\Application\CMSApplication->execute() JROOT/administrator/includes/app.php:61
26 require_once() JROOT/administrator/index.php:32

hello check this plugin
1 () JROOT/plugins/editors-xtd/upagebutton/upagebutton.php:17
i think its not j4 compatible