Support for Windows Phone 8.1 ended 4 months ago, so your phone will no longer receive things like security updates. However, based upon the message you're seeing, it appears that Microsoft have withdrawn the facility to install apps too.
If you've got one of the phones listed below you can upgrade it to Windows 10 Mobile. (Otherwise it might be time to look for an Android phone!):
Lumia Icon, 1520, 930, 640, 640XL, 730, 735, 830, 532, 535, 540, 635 1GB, 636 1GB, 638 1GB, 430, 435, BLU Win HD w510u, BLU Win HD LTE x150q, MCJ Madosma Q501