Go to the config.xml file of UNIT4 SBR manager and open it. Usually this file is on the C drive if you are working locally, or on the UNIT4 application server if you are working through the server.
The config.xml file contains a tag with the name "force signing". Change its value to "true" if it is set to "false".
If the "force signing" tag is not present, the following line can be included in the config.xml:
<forcedocumentsigning> true </forcedocumentsigning>
This should be placed above the </defaults> line. After this it should be possible to sign.