How have you been? Hope you had a great Christmas and new year.
First I would ask if there is anything on the SD card at the moment or is it blank?
If you have data on the SD card, can the phone see it using a file explorer like ES File Explorer File Manager (https://goo.gl/48RD2e). If not I would extract the SD card and try reading it on a computer using a memory card reader.
If a file explorer can see the files but you are still getting that there is no free space please check the amount of free space on the SD card via the settings menu then under storage. If you have no or very little (100MB) of free space I would recommend deleting or uninstalling apps that use the SD card as storage. If you don't know what apps take, how much storage space I would recommend viewing this information via the application or application manager under settings. You could also use a third party app called DiskUsage (https://goo.gl/QRdCy5) which shows your storage usage via a box graphic. The larger the box the more space the folder is taking up. It also show you size of the file inside folders without opening them. Very handy. Also please remember that not all apps use the external SD as storage or the app itself can't be moved to the external SD.
If the SD card is blank I would recommend formatting it under the storage options under settings. Sometimes SD cards come pre-formatted in the wrong format or the partition has become corrupt. Always backup your data.
I hope this helps. If not please provide me with more info. The more info you provide the easier it is for me to find a solution for you.
Have a great day/night