If the lookup field is located on the Opportunity, then your Opportunity has exactly one or exactly zero related records. A lookup field cannot point to more than one record, and it doesn't matter how many other records might be considered "related" based on the business semantics of your data unless you build automation to consider those records.
Probably the most common issue with formula syntax is mismatched parenthesis, or missing commas.
Being rigorous with the formatting (even though it won't appear formatted in Salesforce, until you go to edit the formula, that is) can really help make errors more apparent. Consistency and keeping each line fairly short/self-contained are key here. Applying ...
Here is my attempt at your requirement with both a verbose explanation and a minified version:
//First Scenario Where Sales Type is Open Order and TODAY's date is greater than Scheduled Ship Date
TODAY() > Scheduled_Ship_Date__c,
Line_Status__c = 'OPEN'
based on doc you should try ISBLANK
ISPICKVAL([Lead].LeadSource , 'Trig') &&