Problem statement
"Convert to sale " button on POS is disabled and not bale to convert transaction in SALE for quotation.
Root cause
if the "support multiple transaction mode " check box will not checked in enterprise setting "Convert to sale " button will disabled on POS .

NOTE:- Support multiple transaction is not one time configuration we can make changes in it according to our requirement.
Proposed solution
Go to enterprise setting and make changes in "support multiple transaction mode " checkbox.
follow the give path .administration<< enyerprise setting,
As shown in given image.
Once "support multiple transaction" check box will checked "Convert to sale " button will automatically enabled .shown in below screen shot.