I got 55 marks in JEE Adv. 2019. Which IIT can you assure me with mech. Engg? (SC,male).

Chances are low for all top IITs considering your ranks in 20000-25000 range.
You can however have decentchances with following colleges :

RGIPT Rae Baraeli

IIpE Vishakhapatnam

IISER Pune.

For consideration, you can check the previous year cutoffs here:
https://www.google.com/amp/s/www.embibe.com/exams/jee-advanced-cut-off

Dear student.
Greetings.
The colleges you will be eligible to take admission in is based on your rank in JEE advanced, not your marks. To get admission in an IIT college, you need to be in the top 10-13k.
Thank you.
All the best.