Must login first. Go to "Tools", "Options" and then to "Community tab" (screen)
It depends on external VPS you are using ...
Some external VPS providers were banned from the Market, some of them installed some feature which is preventing us to install, and so on.
And the reason depends on the error number in Metatrader journal on that VPS.
Some more (general advice): post #2
It is impossible to know about the reason because there is no any technical information for that especially - no any error number for example.
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As to installing so look at this thread: https://www.mql5.com/en/forum/464268(it is for MT5 but the login of installation/steps are very similar with MT4 for example).

MQL5 Market failed send request downloading product
- 2024.03.11
- jorgb
- www.mql5.com
Hi, Since the last updates I am not able anymore to install my EA's in the MT5 terminal through Purchases...

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I'm currently unable to download a product onto my MT4. I use a VPS for my MT4, and when I log into MQL5 from the VPS to download the product to put on my MT4, the MQL5 website has no graphics, and it doesn't allow me to log in. When I go to MT4 and click on "Login to MQL5.community," it doesn't let me log in there either. If I use a non-VPS browser, I am able to sign in to MQL5 just fine. Has anyone experienced this issue, and if so, can you please tell me how to resolve it so that I can get this product onto my MT4? Thank you!
See attachments for screenshots.