shasum -a 256 on dmg file returns nothing
The command

$ shasum -a 256 /path/to/file


will return the name of the file if it can't find it. E.g.

Edges-MacBook-Pro:Downloads edge$ shasum -a 256 nofilehere
shasum: nofilehere: 


Otherwise, it will display the hash value and the name of the file:

Edges-MacBook-Pro:Downloads edge$ shasum -a 256 theFile
d7f6436a313e17b01af27cf7582f8aa0149554c20d3257133cf7680e25fc287b theFile 


Check where you downloaded the file. If it is in the Download directory, then this will work:

$ shasum -a 256 ~/Downloads/thinkorswim.installer.dmg


If shasum is not displaying the hash value, then either the file isn't where you specified or the file name is wrong.