I am doing a validation to check that only 2 users can be added in the multi users pickers.
Iam getting error on the last line. Please advice.
//ComponentAccessor.attachmentManager.getAttachments(issue).size() >= 1
ComponentAccessor.customFieldManager.getCustomFieldObjects(issue)?.findByName("Multi user picker").size() >= 1
Your code is getting a list of custom fields with the name "multi user picker" and looking at the size of that list.
What you really want to do is get the current value of the field for the current issue try something like
final String customFieldName = "My Custom Field"
def customFieldManager = ComponentAccessor.customFieldManager
def customField = customFieldManager.getCustomFieldObjects(issue).findByName(customFieldName)
assert customField : "Could not find custom field with name $customFieldName"
def customFieldVal = issue.getCustomFieldValue(customField)
def numberOfOptions = customFieldVal.size()