Service tag is only needed while your warranty is in effect. All you need for parts or instructions is the model name and number. Lots of info and help here--  Support for Inspiron 15 5578 2-in-1 | Documentation | Dell US The service tag is in the bios. 
Hello,   I have a Dell Insprion 15-5578 that I have had for years and love and is needing some TLC to keep it going without replacing the entire machine in this crazy time. I am needing to locate the service tag and/or express Service Code that I now know about and realized it peeled off years ago and was tossed I believe.  I saw someone else with this problem and someone at Dell said they could possibly look it up if it was bought through Dell - which it was - to possibly look up this information. Is there another readily available way to access this information again through my computer? I haven't tried BIOS yet as it is not on me at this exact moment but I will try later if this has no response by then.
My Service tag peeled off years ago and I didn't keep it