This might be solved if you right-click the project and choose "Clean". Then rebuild the project.
There can be many reasons for a linker error. The first thing to try is always to "Clean" the project because Delphi can get out of sync with the compiled units. However, if that does not solve it then you can expand the "Linker error" message (or usually it is the adjacent message) and see what caused the linking to fail. For each individual reason there might be another SO question with exactly that problem.