Well. Funny story.... Had a long chat session with Apple and we discovered I had a huge backlog of purgeable data which wouldn't delete. when you access storage from the apple it said about 38GB available which should be enough to do the update... but when you access disk utility, it said I only had 13.74GB which I found strange. I downloaded omnidisksweeper to find out what files taking up the space, deleted what I could from cache files, backups, photos, etc. but still couldn't get the update to work!!!
Tried using internet recovery which seemed to work but once it go to around 38 mins remaining, I will get a error message saying could not install Mac OS. rebooted and got the prohibited sign! luckily all my important data is backed up, I didn't have a heart attack. Machine went into a boot loop! So went back to square one, alt command R and used disk utility to wipe the drive and start over.
Took 3 attempts but it finally worked, I now have 10.15.5. These things happen. I have had my machine for 3 years with no glitch so I am happy that I was able to revive it with minimal error. Long story short, purgeable data can cause an update to fail. make sure your hard drive is regularly cleaned from cache, old backups and unnecessary old update files which take up a lot of space.