You post is fairly vague without any type of computer information except you have a AMD RX550 GPU card.
From what I understand, you installed your GPU card in several computers including yours but had no Video output?
How is your Monitor connected to the GPU card? Are you using an Adapter? If so, make & model?
Make & Model of Monitor?
Are you able to enter BIOS and check to see if your GPU card is recognized in the PCIe slot where you have it installed?
Also in BIOS make sure your GPU card is the Main Display Device and not your APU integrated Graphics if you happen to have a APU installed. You didn't mention what type of processor you have installed.
But if you installed the GPU card in several computers with no video then i suggest your RMA the GPU Card and have it replaced or repaired.