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IFTHIKAR AHMED 7IFTHIKAR AHMED 7 

app created in classic is not available in lightning experience why ?

app created in classic is not available in lightning experience why ?
Devi ChandrikaDevi Chandrika (Salesforce Developers) 
Hi Ifthikar,
1)Check if you  have selected the option show in lighting experience checkbox,and also make sure that the app is upgraded to lightning.
2)In lightning setup->App manager-->goto your app and check if your app type is lightning or not.If the app type is classic, In the dropdown buttons next to you app and click the upgrade button to upgrade your app to lightning.
3) Click edit button next to your app in App Manager
Then click on user profiles in App Settings  and move the required profiles to selected profiles section and click save.Only users with these profiles can access the app in lightning.
May be you have missed the step 3,so you are not able to access app in lightning.
Please refer below links which might help you in this.
https://success.salesforce.com/answers?id=9063A0000019TPCQA2
https://help.salesforce.com/articleView?id=dev_apps_lightning_upgrade_from_classic.htm&type=5 (https://help.salesforce.com/articleView?id=dev_apps_lightning_upgrade_from_classic.htm&type=5)
https://webkul.com/blog/access-salesforce-classic-app-lightning-experience/

Hope this helps you
If this helps kindly mark it as solved so that it may help others in future.

Thanks and Regards