[SalesForce] The formula expression is invalid: Incorrect parameter type for function ‘Year()’. Expected Date, received DateTime

You are using || (pipe sign operator) in process builder formula but it's not allowed so you can do it in this way:

OR(
IF (
OR (ISPICKVAL ([Case].Flagged__c , "C"),
(
AND 
(ISPICKVAL([Case].Flagged__c , "O"),
[Case].Option__c = FALSE)), "High","")),
IF(
OR (
AND (IF (ISPICKVAL ([Case].Flagged__c , "Me"), 
[Case].Option__c = FALSE),

AND (ISPICKVAL([Case].Flagged__c , "Others"), 
[Case].Option__c = TRUE) , "Medium","Low")
)
)
)

The second line is invalid:

ISCHANGED(TEXT([AidTrack_Call__c].Call_Total__c=))  && 

It should be:

 ISCHANGED([AidTrack_Call__c].Call_Total__c)  && 

There should not be any need to convert to text, nor did you need the equals sign (if you had needed it, you would have needed a string on the other side).