It might be a region lock, account restriction, or network issue. Steam could be limiting access to your inventory and market based on your IP or region. Try checking your region settings or contact Steam support.

I tried but it is not showing me any restriction, and the customer support is just giving generic responses

Without knowing the exact error message you are getting, it's only a guess. But it's probably the IP rate limiting measure. Try to obtain a new IP for your connection without the help of a VPN.

How do I get the new IP?
And it says that I have made too many requests whereas I have not opened my inventory for 2-3 weeks

You hit an API quota limit. This quota limit is IPv4 based. If restarting your router doesn't help, you'll have to wait it out.

IPv4 addresses are not unique anymore. As there are more humans (8 billion) than IPv4 addresses (4.2 billion), ISPs have chosen to route users together on an identical IPv4 address. This technique is known as CGNAT.[en.wikipedia.org]

For Valve's inventory and community market API this bundled together group of users using CGNAT or Dual Stack Lite looks like a single user.

If you waited several days to weeks, your only recourse is getting a public IPv4 address. It doesn't matter whether this one is static or dynamic. Ask your ISP on how to obtain one.

If you are behind Dual Stack Lite (shared IPv4 address, dedicated IPv6 prefix), temporarily disable IPv4 on your system after you made sure IPv6 support is enabled in your router to force Steam to use IPv6. Undo this step after you finished using the market or inventory (you will have to redo it every time you use either). Do not use this as a permanent solution because not every service on the internet did implement IPv6 support yet.

If you already have a public also known as dedicated IPv4 address, you should stop spamming inventory actions.