Actionstep General Ledger codes have the option to enforce the linking of the transaction to matter or not.

For example the default setting for the GL code  5-1320 is to require a matter number, if you dont want this requirement then you will need to either code the transaction to a different GL code that does note enforce a matter number or adjust that setting on your General ledger.

How to adjust the setting in the General Ldger

To adjust the GL you will need to go to Accounting > Accounts > Accounts list
 then change the 'Display' dropdown (at the top of the screen) to show MASTER ACCOUNTS. 
Then find the GL code you are coding to (ie  5-1320)  and click on the GL Name to edit it. 

Then in the section that states 'Matter Restriction'  change this to 'Entries MAY (Optional) be linked to a matter' 

then click save and you will be able to post that general ledger