Please check the basics first such as checking your connections (USB or wireless), ensure the computer has been restarted and if necessary do a NVRAM reset per https://support.apple.com/en-us/HT204063 and next, checking with Fujitsu to see if the device is still supported with the version of Mac OS you have.
Please check with Fujitsu and see if they have a driver for the version of Mac OS you have installed, which you are indicating as 10.14. If they no longer support the scanner then your options are to replace the scanner (this is Fujitsu and the other scanner manufacturers strategy) or to use third party drivers. What has been very successful for thousands of users in your situation is using VuScan by Hamrick . They support older scanners and have free trials so you can test out the software before buying it.