License data is cached in your website for 1 week so that your website do not make frequent query to license server. So sometimes it makes confusion when you change something to license (upgrade, renew etc).
So to troubleshoot please follow following step
Make sure your website is listed in Purchase History > View Licenses > Manage Sites. If not add your site there.
Please go to Front End PM PRO > Settings > Licenses
Then remove your license key and save changes, again input your license key and again save changes.
If above steps do not solve your issue and still getting error follow bellow steps
1. Go to Front End PM PRO > Settings > Licenses
2. Input your license and save
3. in address bar add
&input_license_data=front_end_pm_pro end of your url
4. press Enter
5. a new text box will show under license input field. Click
Get Data bottom of that field.
6. copy that data and paste in that input field. Then save.