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CopyToClipboard[] requires a FrontEnd to work.

Proof:

In: Trace[CopyToClipboard["hi"]]

Out: {CopyToClipboard[hi],MathLink`CallFrontEnd(FrontEnd`CopyToClipboard(System`FEDump`makeCopyBoxes(hi))),{{System`FEDump`makeCopyBoxes(hi),hi},FrontEnd`CopyToClipboard(hi)},MathLink`CallFrontEnd(FrontEnd`CopyToClipboard(hi)),Null}

However, you can (sort of roundaboutly) run the shell inside wls and copy it using your systems clipboard features.

#!/usr/bin/env wolframscript
  
string = "word";
RunProcess[{"bash", "-c", "echo " <> string <> "|pbcopy"}]

Print[x];

This copies string to the clipboard. Implemented on macOS. See here for implementations in Ubuntu (thanks to @ConorCosnett).

CopyToClipboard[] requires a FrontEnd to work.

Proof:

In: Trace[CopyToClipboard["hi"]]

Out: {CopyToClipboard[hi],MathLink`CallFrontEnd(FrontEnd`CopyToClipboard(System`FEDump`makeCopyBoxes(hi))),{{System`FEDump`makeCopyBoxes(hi),hi},FrontEnd`CopyToClipboard(hi)},MathLink`CallFrontEnd(FrontEnd`CopyToClipboard(hi)),Null}

However, you can (sort of roundaboutly) run the shell inside wls and copy it using your system's clipboard tool.

#!/usr/bin/env wolframscript
  
string = "word";
RunProcess[{"bash", "-c", "echo '" <> string <> "'|pbcopy"}]

This copies string to the clipboard.
Additionally, Paste[] requires the FrontEnd, but there's a similar workaround. All together (name from @ConorCosnett):

#!/usr/bin/env wolframscript

string = "Programming is fun!";

copyToClipboardFromString[string_]:=RunProcess[{"bash", "-c", "echo '" <> string <> "'|pbcopy"}]
getClipboard[]:=RunProcess[{"bash", "-c", "pbpaste"}, "StandardOutput"]

copyToClipboardFromString[string]
Print[getClipboard[]]

Implemented on macOS. See here for implementations in Ubuntu (thanks to @ConorCosnett).

CopyToClipboard[] requires a FrontEnd to work.

Proof:

In: Trace[CopyToClipboard["hi"]]

Out: {CopyToClipboard[hi],MathLink`CallFrontEnd(FrontEnd`CopyToClipboard(System`FEDump`makeCopyBoxes(hi))),{{System`FEDump`makeCopyBoxes(hi),hi},FrontEnd`CopyToClipboard(hi)},MathLink`CallFrontEnd(FrontEnd`CopyToClipboard(hi)),Null}

However, you can (sort of roundaboutly) run the shell inside wls and copy it using your systems clipboard features.

#!/usr/bin/env wolframscript
  
string = "word";
RunProcess[{"bash", "-c", "echo " <> string <> "|pbcopy"}]

Print[x];

This copies string to the clipboard. Implemented on macOS. See here for implementations in Ubuntu.

CopyToClipboard[] requires a FrontEnd to work.

Proof:

In: Trace[CopyToClipboard["hi"]]

Out: {CopyToClipboard[hi],MathLink`CallFrontEnd(FrontEnd`CopyToClipboard(System`FEDump`makeCopyBoxes(hi))),{{System`FEDump`makeCopyBoxes(hi),hi},FrontEnd`CopyToClipboard(hi)},MathLink`CallFrontEnd(FrontEnd`CopyToClipboard(hi)),Null}

However, you can (sort of roundaboutly) run the shell inside wls and copy it using your systems clipboard features.

#!/usr/bin/env wolframscript
  
string = "word";
RunProcess[{"bash", "-c", "echo " <> string <> "|pbcopy"}]

Print[x];

This copies string to the clipboard. Implemented on macOS. See here for implementations in Ubuntu (thanks to @ConorCosnett).

CopyToClipboard[] requires a FrontEnd to work.

Proof:

In: Trace[CopyToClipboard["hi"]]

Out: {CopyToClipboard[hi],MathLink`CallFrontEnd(FrontEnd`CopyToClipboard(System`FEDump`makeCopyBoxes(hi))),{{System`FEDump`makeCopyBoxes(hi),hi},FrontEnd`CopyToClipboard(hi)},MathLink`CallFrontEnd(FrontEnd`CopyToClipboard(hi)),Null}

However, you can (sort of roundaboutly) run the shell inside wls and copy it using your systems clipboard features.

#!/usr/bin/env wolframscript
  
string = "word";
RunProcess[{"bash", "-c", "echo " <> string <> "|pbcopy"}]

Print[x];

This copies string to the clipboard

CopyToClipboard[] requires a FrontEnd to work.

Proof:

In: Trace[CopyToClipboard["hi"]]

Out: {CopyToClipboard[hi],MathLink`CallFrontEnd(FrontEnd`CopyToClipboard(System`FEDump`makeCopyBoxes(hi))),{{System`FEDump`makeCopyBoxes(hi),hi},FrontEnd`CopyToClipboard(hi)},MathLink`CallFrontEnd(FrontEnd`CopyToClipboard(hi)),Null}

However, you can (sort of roundaboutly) run the shell inside wls and copy it using your systems clipboard features.

#!/usr/bin/env wolframscript
  
string = "word";
RunProcess[{"bash", "-c", "echo " <> string <> "|pbcopy"}]

Print[x];

This copies string to the clipboard. Implemented on macOS. See here for implementations in Ubuntu.