You cannot install an SSD in a memory module socket, no. If this is the current Inspiron 3521 (from 2022), it will take one M.2 NVMe SSD and one 2.5" SATA SSD.   If it's the 3521 from about 10 years ago, it's 2.5" SSD only.  
I have a Inspiron 15 3521. I’m planning to upgrade my ram. I wanted to know could a sa400 m.2 2280 be seated in the ram slops? My reason for asking is I’m replacing my HDD Sata 2.5 with a 240g a400 Sata 2.5 ssd. Which I understand will be a compatible switch!  
HDD hard drive vs SSD and compatibility